596 E. W. MACBRIDE. 



tion cavity. As invagination proceeds, the future dorsal sur- 

 face of the embryo becomes recognisable by the close apposi- 

 tion of the layers of ectoderm and endoderm which subsists 

 here, whilst on the other side the outer and inner layers of the 

 gastrula diverge from one another (at the point marked o in 

 fig. 7, for example). A little later we are able to perceive 

 that the future dorsal surface has become definitely flattened, 

 this being the first preparation for the formation of the neural 

 plate, the rudiment of the central nervous system ; and as the 

 appearance of this structure enables us to determine the long 

 axis of the future animal, we are able to say that the blastopore 

 is at first posterior. At the same time the peculiar character 

 of the dorsal lip at this stage, with the abrupt transition from 

 ectoderm to endoderm, and the close parallelism of the two 

 layers enables us with certainty to identify it with the corre- 

 sponding part in earlier stages. 



Thus I regard the gastrulation as a fairly uniform pushing 

 in of the under or flattened surface of the blastula, accom- 

 panied by division and multiplication of the cells, such multi- 

 plication being at first most active in the dorsal (future 

 anterior) lip of the blastopore. The blastopore, which is still 

 wide, becomes rapidly narrowed by the upgrowth of the 

 ventral lip (PI. 43, figs. 8, 9, and 10) : in contra-distinction to 

 what Hatschek (3) asserts, the dorsal lip remains relatively 

 stationary. 



Coincidently with this increased activity of growth in the 

 ventral lip, a sharp, abrupt transition becomes now observable 

 in it from ectoderm to endoderm, a fact which supports the 

 explanation given above of a similar phenomenon observed in 

 the dorsal lip. In support of his view that there is an invagi- 

 nation ,of ectoderm round the dorsal lip of the blastopore, 

 Lwofi" speaks of a frequent accumulation of cells just inside 

 the blastopore on the dorsal side, and figures two longitudinal 

 sections of gastrulse iu illustration of this point. I have to 

 definitely state that no such appearances are ever seen in 

 properly orientated sections, and that Lwofl" has been misled 

 by his inability to distinguish between oblique and sagittal 



