40 DEAN. [Vol. XI. 



a somewhat opaque tract of cells, whose anterior end, broadening 

 widely, represents the cephalic plate, and whose obscure dusky 

 line, perpendicular to the rim of the blastopore, is the anlagc 

 of the hinder neural axis. The blastopore is now circular ; it 

 is only at a stage represented in Fig. 49 that any indication of 

 an indented rim has occasionally been found. In a slightly 

 later stage are to be noted changes in the shape and roof of 

 the segmentation cavity, in the growth of the dorsal lip and 

 the rim of the blastopore, and in the coelenteron in its dorsal 

 region. The dilated region of the segmentation cavity has 

 become deeper, its anterior and posterior margins closer to- 

 gether and more sharply marked. The roof of the dilated area 

 has thickened and is irregularly pigmented. While the ventral 

 lip of the blastopore has remained as in the preceding stage, 

 the dorsal lip, PI. IV, Fig. 65, has increased greatly in length, 

 now enclosing about 130° of the egg's circumference; in thick- 

 ness, however, it has remained as in the last figure, except at 

 the blastopore's rim. In this entire region an increase in cell 

 material has become marked, least noticeable at the ventral 

 and greatest at the margin of the dorsal lip. Pressure against 

 the yolk plug seems here to have been reasonably the cause of 

 the inflected rim. The coelenteron below the dorsal lip of the 

 blastopore is notably deeper (ecto-entad) than in the former 

 stage, its fundus is more broadly rounded, and its recessus im- 

 mediately below the dorsal lip, i.e., Kupffer's vesicle, is deeper 

 and sharply trench-like. In the figure the limits of the germ 

 layers are shown ; the inner layer is here thin, but in the axial 

 line becomes the thickened anlage of the notochord. The 

 thickening of the outer layer which caused the appearance of 

 the embryo is found to be exceedingly slight and can only be 

 seen satisfactorily in transverse sections. The Deckschicht, 

 which was earlier noted, cannot be satisfactorily distinguished 

 in this stage. 



Further growth of the embryo is illustrated in PI. Ill, Fig. 51. 

 Here the cephalic region is seen to have spread out, ovate in 

 outline, as if flattened upon the rounded surface of the egg. 

 The spinal axis is prominently marked, and at its hinder end 

 communicates with coelenteron ; a small light-colored band 



