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G. R. DE BEER 



the structures anterior to it. In transverse section the hypo- 

 physis about this stage appears as in Text-fig. 31. At first 

 sight it looks as if the hypophysis must have been derived 

 in situ from the underlying endoderm cells. But careful 

 observation with high powers reveals the differences that have 

 been met with before between the cells of the hypophysis and 

 •the endoderm cells, viz. absence of yolk and different orienta- 

 tion. In many cases the limiting membrane of the gut-roof 



Text-fig. 31. 



Text-fig. 32. 



Amia calva. The same as Text - 

 fig. 29. Transverse section. 



A m i a c a 1 V a , 8 mm. long. Trans- 

 verse section. Begiiming of the 

 hypophysial cavity. 



appears to be continuous with that of the hypophysis, and this 

 fact puzzled me for some time, but I believe it to be without 

 significance and due to the close apposition of the hypophysis 

 to the gut-roof. In some sections the real discontinuity of the 

 membrane can be seen. The hypophysis is still in contact 

 with the gut-roof, and at this stage the beginning of the hypo- 

 physial cavity can be observed (Text-fig. 32). Mesenchyme 

 begins to make its way between the floor of the brain and the 

 gut-roof, and in the next stage (Text-figs. 33 and 34), where 

 the hypophysis has separated off the endoderm, it is enclosed 

 by a layer of mesenchyme. 



