EMBRYOLOGY OF THALASSEMA MELLITA 195 



II. Gastrulation to Trochophore. 



I . Gastrulation. 



The process of gastrulation begins as a rule about seven 

 hours after fertilization (PI. II, Fig. 21). It is a modified 

 embolic type and consists merely of the insinking of the ento- 

 blastic plate (Text-Fig. 4, A and B^. I could not discover any 

 infolding of cells such as Conn i^^(i) describes. The entoblastic 

 plate consists, as has already been indicated, of eleven cells, just 

 as is the case in Aricia, Ainpliitritc, Clyinciiella^ Arenicola and 

 Podarke. Although, in T/ialasscjna, the order in which the 

 cells leave the surface is subject to variations, the macromeres 

 generally sink in first, next the fifth quartet and finally the fourth 

 quartet (PI. II, Figs. 21 to 24). As gastrulation proceeds there 

 is a flattening at the lower pole and a rounding out of the wall 

 of the praetrochal region (Text-Fig. 10, D). All of the ento- 

 blast cells become greatly elongated and the nuclei lie in their 

 swollen inner ends. These cells sink in so far that some of them 

 come in contact with the rosette region and the lateral ecto- 

 blastic wall. A little later the endoblastic mass rounds out and 

 withdraws from the body wall, but the points of contact are still 

 preserved by protoplasmic strands (Text-Fig. 8, F) which, owing 

 to the viscid nature of the protoplasm, are drawn out. These 

 threads are, accordingly, of the same nature as those occurring 

 at the surface of the tgg immediately after fertilization and are 

 not due to spinning phenomena. It seems probable that a simi- 

 lar explanation may be given for the strands described by Tread- 

 well ('01). 



2. Closure of the Blastopore. 



The blastopore, when first marked out, lies exactly at the 

 lower pole of the ^gg, but it soon shifts slightly toward the 

 future ventral side (Text-Fig. 4, C). It is bounded at the pos- 

 terior end by the apex of the X group, at the sides posteriorly by 

 small cells of the third quartet, c and d quadrants, and anteri- 

 orly by cells from the second quartet r, a and b quadrants (PI. 

 II, Fig. 23). Later the sides of the blastopore become lined 



