47© Journal of Comparative Neurology and Psychology. 



somite is connected with both the notochord and the median undiffer- 

 entiated mass. The archenteron is drawn out anteriorly to a very 

 slender pointed cavity (Figs., o, 4, 5) which ends beneath a depres- 

 sion in the floor of the brain usually called by authors the infundib- 

 ulum. As this is not the infundibulum I shall for the present refer 

 to this part of the brain as the ' 'infundibulum," using quotation 

 marks. Following a series of transverse sections forward one sees 



Fig. 5. Squalus ac, 15 somites, medial view of a model of the right half 

 of the head. ** part of neural tube which is still open. x --J- 



ncliV 



Heura 



I pl-^te 



Fig. G. Sqnalus ac, Balfour's stage D. sagittal section of anterior end. 

 X 75. 



that as the narrow archenteron disappears, the proper entodermal 

 wall fuses with the overlying median mass and thence continues 

 forward beneath the ^'infundibulum" as a solid mass of preoral 

 entoderm. At this time the neuropore and dorsal seam of the 

 neural tube are not yet closed and the preoral entodenn extends up 

 to the point where the neural plate passes over into ectoderm. These 

 relations are properly shown in deal's figs. C and D. For some time 

 the anterior part of the median mass and the preoral entoderm cause 



