The origin of the embryo-cell is still in volved in obscurity. Most ovologists are dis posed to connect it in some way or other with the previously existing germinal vesicle, or some part of its contents, and more especially the nucleus. For the solution of this ques tion, as already remarked, a more accurate knowledge of what happens to the germinal vesicle at the time of that disappearance which has been so commonly observed at the period of the maturation of the ova of almost all ani mals, will be required. Does the macula re main, as has been alleged by some, to form the nucleus or the whole of the embryo-cell ? Or, in other cases, if the multiplied maculm are dispersed among the granules of the ger minal yolk, are they again collected together into a mass or spherule to form the embryo cell ? Or, again is the embryo-cell formed out of other materials, and not neces,arily either partially connected with, or wholly de rived from, the germinal vesicle ? And finally, might it not be, according to some recent ob servations, such as those of J. Miller on En tochoncha and those of Remak on the frog, that the disappearance of the germinal vesicle is not attended with the dispersion of its con tents, but is a phenomenon caused only in a certain number of animals by the solution of the delicate external wall of the vesicle, and by some change in the position and consist ence of its contents ? Further observations will be required to determine this point ; but if in the meantime we regard it as most pro bable that the embryo-cell is in some way or other connected in its origin with the germinal vesicle, we might further found upon this the speculative view that the blastodermic cells and the blasterna from which unques tionably, by a histogenetic process of cell-di vision and multiplication, the various textures and organs of the animal body are produced, may be regarded as the descendants of the original cell-germ frona which the ovum was developed combined with the sperm. We should thus trace the organic cellular connec tion between the succession of parents and offspring, which I have stated to be one of the most general facts in organised nature.
The observations respecting the very re markable movements of the yolk, before and during the earlier stages of the segmenting process which have now been recorded by several physiologists, must excite the liveliest interest, and suggest subject for much reflection as to the evidence they may. afford of the causes of this change, or, if we may use the expression, of the forces by which segmenta tion is brought about. There seems to be little doubt that the embryo-cell (and its nu cleus first of all) is the earliest to become di vided, and that the process of cleavage then proceeds from the surface of the segmenting mass inwards towards the cell ; but in what relation the nucleus, granular substance of the yolk, and ovicell-membrane stand to each other in this process, must be left to be de termined by future researches.
Of the other early changes in the ovum which immediately follow fecundation and precede embryonic development little need here be said. They consist principally in the
greater degree of consolidation and compact ness acquired by the germinal part of' the yolk, and in the formation in most animals of a clear space between the surface of the yolk substance and the enclosing vitelline mem brane. It is in this clear space, or, as it has been called by Newport, respiratory chamber, that the spermatozoa have been observed in those instances in which they have been as certained to penetrate into the cavity of the ovum. There is another phenomenon of the same period, which has now been so frequently observed, and which is of so peculiar a nature, that it must not be passed over without no tice ; I allude to the appearance in the re spiratory space of one or more clear and highly refracting spherules, nearly of the size of the germinal vesicle, but quite independent of it. These clear hyaline-like globules have been observed in the ova of Gasteropodous Mol lusca after fecundation by almost all those who have attended to the ovology of this class of animals, among whom may be mentioned Dumortier, Pouchet, Van Beneden, Nord mann, and Vogt ; in the Annelida by Quatre fages ; in Maminalia by Bischoff and Barry ; and in Batrachia by Newport. From the observations of Quatrefages in Hermella they appear to be excluded or expressed, as it were, from the clear basement-substance of the yolk ; and Bischoff states that they gra dually disappear, or are dissolved without obvious change. We are at a loss to deter mine what office these globules may have in connection with the changes of the ovurn at the time they appear.
Lastly, I would notice the interesting re lation which appears to subsist between the situation of the germinal vesicle and the cen tre of the germinal membrane afterwards formed, or the germinal pole of the ovum, and the conformity in the direction of the line of the first cleavage of the yolk with that of the principal axis of the embryo in verte brate animals. The first fact has been observed in all animals, and the latter has been ascer tained by Mr. Newport's researches in Ba trachia, and by observations which I have myself made in the bird's egg during its de scent through the oviduct. These facts, as yet inscrutable in their nature, point to in. teresting laws relating to the connection of the first phenomena of development, which may be worked out by the further prosecution of these inquiries.
In the preceding part of this article we have considered chiefly the anatomical struc ture of the ova of animals, and have made little mention of their chemical composition. The knowledge of the latter subject is as yet very imperfect. In a recent Memoir* Messrs. Valenciennes and Fremy have given an ac count of an extended series of experimental researches in which they have been engag,ed, with a view to determine the differences in the chemical composition of the ova of dif ferent animals, and although this investigation is still necessarily' incomplete and fragmentary, they appear already to have arrived at some interesting results. The following are some of the more important of these results.