HUMAN EMBRYO OF THIRTEEN SOMITES 32 J 



NERVOUS SYSTEM 



The nervous system has not proceeded very far in its differ- 

 entiation. The brain flexures do not agree with the His models 

 of this stage, but correspond more to the older embryos described 

 by Thompson ('07) and van den Broek ('11). The most distal 

 portion representing the forebrain is still open and is bent almost 

 at right angles to the mid-brain. The long axis of the fore- 

 brain lies in a cephalo-caudal plane and almost parallel with the 

 long axis of the hind-brain. The most cephalic point of the 

 nervous system is thus represented by the junction of the fore- 

 brain and the mid-brain. Near the caudal extremity of the fore- 

 brain there is a thickening together with an evagination of the 

 brain ectoderm. This evagination is almost in contact with the 

 ectoderm of the stomodeum and undoubtedly represents the in- 

 fundibulum. Cephalad to the infundibulum and about in the 

 middle of the lateral expansions of the cephalic neuropore there 

 is a slight depression of the ectoderm on each side which repre- 

 sents the beginning of the optic vesicles. 



The mid-brain is quite extensive as is apparent from an exami- 

 nation of figure 3. Its floor is smooth and exhibits a thickening 

 at the cephalic end. Caudally there is a flexure of the floor 

 between the mid-brain and the hind-brain. The floor of the 

 mesencephalon is thickened at its cephalic end. The trigeminal 

 ganglion is present as a distinct mass of cells. Its position is 

 represented in figure 3 by a broken circle. The hind-brain passes 

 gradually into the spinal cord. A distinct neck flexure is not 

 present. 



The medullary tube has its greatest diameter at the cephalic 

 extremity. It diminishes gradually in size caudally. At the 

 caudal neuropore it exhibits a slight enlargement. 



DIGESTIVE SYSTEM 



The stomodeum is a broad and deep invagination of the ecto- 

 derm between the heart bulging and the head. It touches the 

 entoderm of the pharynx and forms with it the beginning of an 

 oral plate. There is no indication of an hypophysis. The ecto- 

 derm lining the stomodeum is thickened especially in the roof. 



