38 DEAN. [Vol. XI. 



In PI. Ill, Figs. 48 and 49, are figured two later gastrulas, 

 and vertical sections of those stages are given in PI. IV, Figs. 

 63 and 64. Outwardly these stages arc conspicuous, on ac- 

 count of the sharp color contrast they present in their unpig- 

 mented and pigmented zones ; the vegetative pole of the o,^^ 

 is seen to darken in color as it becomes reduced in outward 

 size by the constricting blastopore. The uppermost part of the 

 embryo is in these stages somewhat depressed, and is slightly 

 darkened by pigment. In the earlier gastrula the dorsal lip of 

 the blastopore is more sharply drawn, — shown in the left of 

 the figure, — while in the later form the dorsal lip is often indi- 

 cated by a slight nick-like indentation of the rim of the blasto- 

 pore. The thickening of the dorsal lip in the median plane, 

 which shortly appears, is the first indication of the embryo's 

 axis. Examination of sections of these stages enables us to 

 better understand the advances which have taken place in the 

 development. In the earlier gastrula the blastopore's dorsal lip 

 has separated from the yolk cells and has enclosed about 45° of 

 the egg's circumference ; it is thicker comparatively than that of 

 Lepidosteus, and already includes the three germ layers which 

 are seen to become confluent near the margin of the lip. Its 

 exact mode of growth can of course only be determined ex- 

 perimentally, but from the blunted end of the coelenteron it 

 appears that to some degree a growth of the entire lip has 

 taken place. The segmentation cavity has greatly flattened, 

 to a degree in fact which renders it difficult to be determined in 

 the anterior region of the embryo ; its floor has also extended 

 and flattened, and its four or five layers of loosely associated 

 cells are often seen clearly distinct in many regions from the 

 yolk mass. As yet only a trace of the anterior lip of the bias-, 

 topore has appeared ; here the cells of the lower hemisphere 

 are seen to increase in size and irregularity in the direction of 

 the dorsal lip. In this region pigment occurs plentifully, and is 

 present in notable quantity in the cells lining the coelenteron. 



The older gastrula may be directly compared with that of 

 Lepidosteus, PI. II, Fig. 31. It presents a marked advance in 

 the growth of the blastopore ; its dorsal lip now surrounds a 

 quadrant, and its ventral lip about 20° of the egg's circumfer- 



