End of Third Week.
By the end of the third week or commencement of the fourth the em bryo has undergone important changes. Figs.111 and 112 show the whole ovum and the embryo at this age.
The whole ovum, which is shown of the natural size in Fig. 109, is somewhat pyriform, and measures 1 inch along its greater diameter; it is covered all over with long branched villi. The embryo, which is repre sented on a larger scale in Fig. 112, has increased very considerably in size. It is no longer straight, but is bent very strongly on itself—so much so, indeed, that it forms more than a complete circle, the flexure of the whole embryo being quite as strongly marked as in a rabbit embryo of the twelfth day (Fig. 97).
The yolk-sac is about the same size as before, having a diameter of .11 inches, but its surface is wrinkled, and it is now connected with the em bryo by a very distinct stalk. The amnion is very closely applied to the embryo, and the allantoic stalk, as before, connects the embryo with the chorion.
In the embryo itself the most noteworthy features besides the strongly marked flexure are the following: There are indications of thirty-five protovertebrEe, the greatest number ever present. Both pairs of limbs are present as short buds with very wide bases of origin, the arm and leg of either side being connected together by a low ridge, the Wolffian ridge, of which the limbs are merely special local developments. All the main divisions of the brain are present, and can be easily recognized, as can also the ganglia of several of the nerves. Five visceral arches are visible on each side—viz., the maxillary (forming the upper jaw), mandibular (forming the lower jaw), hyoidean, and first and second branchial arches.. The optic vesicles are present as outgrowths of the brain, but there is as yet no trace of the lens. The alimentary canal is a nearly straight tube, which communicates with the yolk-sac by only a very narrow channel.
The whole embryo measures about .15 inches along its longest diameter, but, owing to the flexure of the body, its real length must be at least double this.
Iburth Week.
By the end of the fourth week the rudiments of all the more important organs have become definitely established, and the embryo has arrived at a very well-marked period of development. It has now reached a stage corresponding closely with that attained by the rabbit embryo about the twelfth day, and by a chick embryo towards the end of the fourth day of incubation.
We have already had occasion, when considering the earliest stages of development, to notice the extreme slowness with which the human em bryo develops. This is very strikingly exemplified by the facts just stated—viz., that the human embryo takes four weeks to reach the same stage of development and complexity of organization, and, what is more, the same actual size, that a chick embryo accomplishes in exactly one seventh of the time.
This is doubtless in part due to the fact that the chick embryo is ex ceptionally well off in having an enormous supply of food ready to hand in the shape of the yolk of the egg; while, on the other hand, the mam malian embryo has to devote part of its energies to the establishment, at as early a period as possible, of the placenta for the sake of obtaining nutriment from the mother. But although this may explain why the mammal develops more slowly than the chick, it does not in any way help us to understand why the human embryo develops during its early stages at less than half the rate of the rabbit, and we must be content for the present to accept as an unexplained fact that the human embryo does dawdle over its development in a manner yet completely inexplicable.
As the stage we are now dealing with, the end of the fourth week, is an important one in many ways, and as our knowledge of it, owing to His's admirable investigations, is in a very satisfactory and fairly complete state, it will be well to describe it in some detail.