CH. VI] 



FORMATION OF THE GERM-LAYERS 



67 



which the dorsal lip has begun to roll. We find, at the upper 

 end of this crevice, the pigmented ends of those cells that were 

 previously at the surface. During later stages the space, which 

 we may at once speak of as the archenteron, becomes longer, 

 due to a further progression of the dorsal lip over the Avhite 

 hemisphere. If the 

 section were taken 

 somewhat to one side 

 of the median line, 

 the length of the ar- 

 chenteron would be 

 found to be less than 

 in the median line, 

 because the rolling in 

 has been relatively 

 less. If we make a 

 section at right angles 

 to the last in the plane 

 Y-Z, in Fig. 19, A, 

 we cut the two horns 

 or ends of the cres- 

 cent. The Cavity On FIG. 25. A (small figure inside B). Longitudinal 

 ipVi dirlp ic -inct KP section through young embryo. B. Cross-section 



s 3 Ul of last. (After Schultze.) 



ginning, owing to the 



smaller amount of closing in from the sides of the lateral lips 



of the blastopore. (Fig. 19, B.) 



A section at right angles to the last section in the plane of 

 the line in Fig. 25, A, is shown in Fig. 25, B. The archen- 

 teron is seen in the upper part of the section. Its upper or 

 dorsal wall is made up of small cells, while its floor is formed 

 of large cells filled with yolk. The segmentation-cavity fills 

 the centre of the section. 



During the time when the yolk-plug is withdrawing from 

 the surface, the segmentation-cavity becomes smaller, owing, 

 without doubt, to the intrusion of the large yolk-mass into its 

 interior, and finally, when the archenteron begins to open, the 

 segmentation-cavity is almost entirely obliterated. The seg- 

 mentation-cavity is thus utilized by the embryo, for into this 

 cavity is pushed the yolk-mass as the latter is overgrown by 



