434 A Three Weeks' Human Embryo 



9, 10. Mesencephalic. — The roof of the mesencephal or mid-brain is 

 in this specimen so broken that details of form were impossible to work 

 out, but in general, it is possible to see that there are two pairs of total 

 folds, one, the 9th, beginning in a mesal pocket lying caudad of the deep 

 notch dividing the diencephal from the mesencephal, or in other words, 

 candad of the future post-commissure and extending obliquely caudad 

 for half the length of the mesencephal, the other, the 10th, arising near 

 the cephalic border of the metatela and ending abruptly near the point 

 where the IVth nerve will take its origin (4 of Fig. 10). 



12. Cerebellar. — These are two total folds represented clearly only in 

 Fig. 11, rising at the cephalic border of the metatela and involving the 

 part of the lateral wall which at this stage of development represents the 

 cerebellum. Their history has not been traced. 



lJf-18. Oblongata,. — Are folds which arise at the edge of the mem- 

 branous roof of metatela, and extend across the brain wall near to the 

 ventrimeson. If one remembers that the neural plate on closing in this 

 region as well as in the fore-brain, at first, is a tube with as thick walls 

 dorsally as elsewhere, it is easily seen how the origin of these folds at 

 the edge of the membrane, may represent the dorsi mesal pockets occur- 

 ring farther cephalad, especially since Locy's '° work shows marginal folds 

 in the early stages. 



13. Oblongata 1. — In this and several other specimens studied, this 

 fold seems fully separated from the following, obi. 2, but the sections are 

 so cut as to make it difficult to trace it with certainty to the dorsal edge. 

 In other human specimens studied I was not certain of its presence. In 

 this region some authors find a ventral representative of the cerebellum, 

 but in this specimen, at least, there is no such relation. 



IJ/.. Oblongata 2. — This fold is one of the most strongly defined of the 

 series. Its invariable connection with the roots of the Vth nerve and the 

 Gasserian ganglion makes it a land-mark in the embryos of all vertebrates 

 studied. The Gasserian ganglion is large but loosely formed and pene- 

 trated by branches of the jugular vein. 



15. Oblongata S. — This is also a sharply defined fold and has been 

 widely recognized though as yet no structure has been definitely associated 

 with it. 



16. Oblongata k. — The roots of the Yllth and Vlllth nerve are as 

 invariably associated with this fold as the Vth with its fold. In the 

 present specimen, in its dorsal portion, it is divided into two folds, the 

 more cephalic being connected with the roots of the Vllth, the more 

 caudal with those of the Vlllth nerve. The roots are very short, soon 

 uniting with their corresponding ganglia. The ganglia lie close together 



