EMBRYOS OF THIRTY-ONE TO THIRTY -TWO DAYS. 



149 



tion of the neck-bend, the growth of the limbs, the narrowness of the opening of 

 the sinus cervicalis, the elongation of the umbilical cord, and the expansion of the 

 hind-brain are all evidences of advancing development (compare Fig. 95). From 

 the distal end of the umbilical cord springs the amnion, beyond which there passes 

 out from the cord the narrow stalk of the yolk-sac. The cavity in the interior of 

 the cord is a continuation of the ccelom of the embryo and through it the yolk- 

 stalk takes its course. 



Figure 98 is very instructive, for it represents an embryo which, although 0.2 mm. 

 shorter than the one shown in figure 97, yet is much more advanced in development, 

 as is evidenced strikingly by the enlargement of the whole 

 . head and the elongation of the limbs and the demarcation 

 of the hand from the rest of the anterior limb. The 

 orifice of the cervical sinus is narrow. On the ventral 

 side of the anterior limb, the body shows three rounded 

 eminences corresponding to the auricle of the heart, the 

 ventricle of the heart, and .the liver. 



Embryos of Thirty-one to Thirty-two Days, 10 to 12 

 mm. As typical specimens of this stage we may take two 

 embryos, one of 10.0 mm. (Figs. 99 and 100), the other 

 of 11.5 mm. (Fig. 101). Figure 99 shows the embryo, 

 Series 1000, the chorion and amnion having been opened; 

 the embryo lies somewhat obliquely on its left side, therefore 

 figure 100 has been added to give a correct profile compara- 

 ble with the other figures of the series. As compared with the previous stage (Fig. 

 97), the back has straightened out somewhat, though the lower end of the body 

 is still rolled over. The head has risen and increased considerably in size. Be- 

 tween the end of the region of the hind-brain and the level of the arm the dorsal 

 outline has become slightly concave. This concavity His designated the "Nacken- 

 grube." The first gill-cleft, owing to the completed closure of the cervical sinus, is 

 the only one visible externally.' It is the anlage of the external auditory meatus. 

 It is separated from the mouth by a prominent mandibular arch. On the cephalic 

 side of the mouth the maxillary process has become more prominent, but the two 

 portions of the maxilla do not yet meet in the median line. The primitive seg- 

 ments are still marked externally. The limbs show indications of their tripartite 

 division, the fore-limb being more advanced than the hind-limb. The division of 

 the digits of the hand is just indicated. The abdomen -bulges out, owing to the 

 growth of the liver. There is a true tail, which is now near its maximum develop- 

 ment. The umbilical cord has lengthened and shows the commencement of its 

 spiral twisting. The amnion springs from the end of the cord, leaving only a short 

 stretch of the body-stalk between the cord proper and the chorion. The amnion 

 envelops the embryo closely. In embryos slightly older than these the changes in 

 form above mentioned have progressed further. The body is straighter, the head 





FIG. 98. HUMAN EMBRYO OF 

 9.4 MM. SERIES 1005. 

 X 5 diams. 



