DEVELOPMENT OF THE NERVOUS SYSTEM 525 



of the entoderm to the ectoderm was so close that there appeared 

 to be a fusion of the two. This form, also, is not the most favor- 

 able for the study of this problem because cavities and separa- 

 tions which would be present in many other forms are not apparent 

 here. 



Figure 7 shows a median sagittal section of an embryo in an 

 early Stage VIII. By the cephalic growth of the embryo the 

 space (s) is formed, beneath which is the ectoderm covering the 

 yolk mass. Above this space is the outer layer of the ectoderm 

 and then the stratum basale (nervous layer of ectoderm). No 

 indication of a thickening of the stratiun basale can be seen to 

 indicate the formation of the hypophysis. Figure 8 is a sagittal 

 section of an embryo in a later Stage VIII. The rudiment of the 

 hypophysis is seen as a thickening of the stratum basale. The 

 ectoderm blends so with the entoderm that it is difficult ta 

 determine the boundaries of each. 



Figure 9 is a cross-section through the hypophysial anlage of an 

 embryo in a Stage VIII. This is the earliest series in which I 

 could recognize the anlage of the hypophysis in cross-section. It 

 appears as a medial thickening and shows no indication of a paired 

 origin as described by Gaupp ('93), Pollard ('94), Lundborg ('94), 

 Gregory ('93), and Kingsley and Thyng ('04). Gaupp's models 

 showing this condition are particularly instructive. 



Figures 27 and 28 show that laterad to the hypophysis there is a 

 thickening and a folding of the ectoderm. This lateral thickening, 

 however, is entirely distinct from the true hypophysial ingrowth 

 and does not enter into the hypophysis. 



Figure 37 shows a median sagittal section of a larva in Stage IX. 

 The hypophysis has increased in size and is still very intimately 

 associated with the entoderm. It is broadly connected with the oral 

 ectoderm. Figure 10 shows a transsection through the posterior 

 portion of the hypophysis in an embryo of the same age as 

 figure 39. 



By Stage X (fig. 38) the distance between the oral ectoderm 

 and the hypophysis has increased greatly, due to the cephalic 

 flexure and the growth of the embryo. The hypophysis, however, 

 is still connected by a well marked double strand of cells with the 



