Third Month.
At the end of the third month the embryo, which from this time is commonly spoken of as the fcetus, is from 5 to 5f inches in length, and weighs 1500 to 1875 grains. The head is still very large relatively to the rest of the body, but not nearly so much so as in the earlier stages. Both mouth and eyes are closed. The neck, already present at the eighth of ninth week, is now far more evident, and the limbs, though still small, have acquired their definite shapes and proportions; the nails are prose& as very thin plates. The integument is slightly firmer than before, but is still very thin, transparent, and rose-colored. Hitherto, part of the alimentary canal has been situated in the allantoic stalk, and therefore outside the embryo (Fig. 114), but by the end of the third month this is withdrawn, and the whole alimentary canal, which has increased greatly in length, is thenceforward situated entirely within the abdominal cavity.
Nervous Sysletn.—The cerebral hemispheres are large, but do not yet cover the mid-brain, which latter is smooth and presents no trace of its subsequently acquired division into the corpora quadrigemina. The cerebellum is a broad transverse band; the fourth ventricle is a largo cavity with a very thin roof; and the spinal cord presents well-marked brachial and lumbar enlargements.
Urino-genital•Organs.—By the end of the third month very important changes have been effected in the mutual relations of the rectum, bladder, and urinary and genital ducts—changes which result in the establishment of the external generative organs and in the external differentiation of the sexes. Though these changes commenced at an earlier period than that with which we are now dealing, it has been convenient to postpone their description until we were in a position to deal with them in their entirety.
The condition of the parts with which we are concerned about the end of the fourth week is shown in Figs. 114 and 115. The terminal portion of the intestine is dilated to form the cloaca, Cl. Into the cloaca open— (1) on the ventral surface, the cavity of the allantois, All, which later on becomes the bladder; (2) on the dorsal surface, nearly opposite the aper ture of the bladder, the two Wolffian ducts, coming from the Wolffian bodies or primitive kidneys.
By the end of the fifth week the following changes have occurred: (1) the essential organs of reproduction have appeared as a pair of longitudinal ridges lying along the inner sides of the Wolffian bodies (Fig. 117); (2) a pair of new ducts, the 111(illerian ducts, have appeared, which open in front into the body-cavity, and unite together posteriorly to open into the neck of the bladder just before it opens into the cloaca; and (3) the Wolffian ducts have shifted so as to open into the neck of the bladder with the Mfillerian and so only indirectly into the cloaca.
During the sixth week the permanent kidneys and ureters appear. The exact mode of their development in man is not known, but it is probable from analogy that the ureters are formed as outgrowths from the dorsal surface of the Wolffian ducts, in which case it is very possible that the saeeular outgrowths seen in this position at the end of the fourth week (vide Fig. 115, U) are their first rudiments. The kidneys are probably formed from two masses of tissue immediately behind the Wolffian bodies and directly continuous with them. The ureters very early acquire in dependent openings into the bladder rather higher up than the open ings of the Wolffian and Mfillerian ducts. From this period up to the ninth week the changes are comparatively unimportant.
By the ninth week the essential reproductive organs have increased greatly in size, while the neck of the bladder has elongated considerably to form a sinus urino-genitalis. The two ureters open directly into the bladder, and the Wolffian and Mfillerian duets into the sinus urino-geni tabs some distance below the ureters. Owing to the development of a median septum, the sinus urino-genitalis and the rectum are almost com pletely separated from one another; they still, however, open to the ex terior by a common cloacal orifice, though the cloaca itself is now a very shallow chamber.