SOCIETY OF tHE UNIVERSITY Of ABERDEEN. 



10? 



by Professor Thompson (10). On the right side there is an infolding 

 of the body wall which produces a ridge running from the anterior 

 part of the dorsal mesocardium to the septum transversum, and 

 slightly constricting off the pericardial coelom from the parietal 

 recess; this may be the "pulmonary ridge " of Mall (8). Only the 

 cranial end of this ridge is present on the left side. 



'-3 Tr. 



Y.V 



Fig. 11. 



Fig. 12. 



Model showing septum transversum viewed from above. Fig. 12 shows right 

 half of septum removed and the right umbilical vein passing in to open 

 into the sinus venosus. 



Sp.c., medullary canal; d.Ao., dorsal aorta; P.K., parietal recess; Am., amnion ; 

 S.v., sinus venosus; A'. TV., septum transversum; Y.S., yolk sac; r.u.v., right 

 . vitelline vein ; F.G., fore-gut ; ///., third somite. 



NERVOUS SYSTEM. 



The medullary plate is open at its head and tail ends almost to 

 the same extent as in Eternod's embryo of eight mesodermic somites. 

 In Kollmann's embryo of fourteen somites the medullary groove is 

 closed behind, but open anteriorly. The brain shows very distinctly 

 the three parts, prosencephalon, mesencephalon, and rhombencephalon, 

 separated by grooves (Plate XIII.). There is no demarcation between 

 the rhombencephalon and the spinal cord, but the junction seems to 

 be at the neck bend at the level of the first pair of mesodermic 

 somites. Just cranial to this point the two folds of the medullary 

 groove have fused. The fore-brain shows elongated, gutter-like 

 evaginations of its later walls which are the commencing optic vesicles. 

 The walls of these vesicles are many cells thick, and are in close 

 contact with the external ectoderm (Fig. 13). From the floor of the 

 fore-brain is a slight out-pushing, the anlage of the infundibulum. 



