No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 425 



Gastriilation ; Fate of the Blastopore. 



The beginning of invagination may be observed when the 

 ^gg contains about 175 cells. The egg at this time is a hollow 

 blastula with comparatively thin walls, especially on the upper 

 side. The apical pole at this time has been pushed forward 

 through an angle of about 45°, and the rotation continues dur- 

 ing the process of gastrulation. For some time before invagi- 

 nation begins the Qgg flattens and the lower pole loses its 

 convexity. The invagination is first seen in the center of the 

 vegetal pole ; the four central cells and the cells of the fifth 

 quartette first sink in, forming a small, round concavity ; as 

 this pit becomes deeper, the cells of the fourth quartette are 

 involved in the process. All of the yolk-laden cells become 

 invaginated, and the stomatoblasts may be seen at the edges of 

 the depression. Doubt has been expressed as to whether the 

 cells called entodermic and ectodermic in the early cleavage of 

 mollusks really prove to be so by their history. In Planorbis, 

 at least, the edge of the blastopore marks quite sharply the 

 boundary between the protoplasmic and yolk-laden cells. The 

 invaginated area is at first nearly circular in outline, but, as the 

 depression deepens, it assumes an elongated form ; and, finally, 

 the mouth of the gastrula becomes reduced to a narrow, slit- 

 like orifice. The length of the elongated blastopore becomes 

 reduced by closure from behind. The anterior end of the 

 blastopore is a comparatively fixed point, and is situated just 

 behind a pair of small cells of the second quartette. PI. XX, 

 Fig. 46, shows a stage in which the blastopore is reduced to a 

 minute slit lying between two cells. Another preparation 

 showed these two cells in contact, so that the blastopore in 

 this species may be said to close, though it is only for a brief 

 period. The oesophageal invagination occurs very soon after 

 the blastopore closes, and at exactly the same place. Its orifice 

 is small and nearly circular, and becomes surrounded by cilia. 

 It gradually deepens and gives rise to a diverticulum on the 

 ventral side, which becomes the pouch of the radula. 



The gastrulation in Planorbis is purely embolic. The cells 

 of the ectoderm do not slip over the edges of the entomeres, as 



