328 Tlios. II. Montgomery. 



lie finds that tlie yolk columns are as numerous as the nuclei, and that 

 at a stage of about 30 cells all the cells reach the surface to constitute 

 the blastoderm, their cytoplasm masses then fusing with the peri- 

 plasm. 



12. Gastrulation and Formation of the Germ Layers. 

 There first forms a thickening of the germ disc at a point slightly 

 posterior to its center, this is the anterior cumulus, and it becomes 

 only slightly elevated above the surface of the egg. It is at first circu- 

 lar, with a shallow circular gastrocoel, and rapid cell proliferation 

 takes place from it. This anterior cumulus proliferates first vitello- 

 cytes, branched highly vacuolated cells that ingest yolk rapidly and 

 some of which sink into the yolk, while most wander along its surface 

 just beneath the germ disc; and second, mesoblast and mesentoblast 

 cells that scatter upon the surface of the yolk. A second or posterior 

 cumulus arises j)osterior to the former and somewhat later in time, 

 usually at the posterior edge of the germ disc ; it is more elevated and 

 prominent, and its thin anterior border marks the future thoraco- 

 abdominal boundary; it has no gastrocoel and proliferates only vitello- 

 cytes. The two cumuli become later connected by movement of 

 vitellocytes between them. Still other vitellocytes arise at the anterior 

 an<I lateral margins of the germ disc. The gastrulation process is 

 accordingly threefold: (1) at an anterior and (2) a posterior cumu- 

 1ns, and (3) at the anterior and lateral margins of the germ disc, 

 from all of which points arise vitellocytes, but only from the first does 

 mesoblast and mesentoblast originate. The layer between ectoblast 

 and vitellocytes seems to have exclusive origin from a group of about 

 eight eells placed at the anterior cumulus, the descendants of these 

 eight cells moving in all directions beneath the germ disc ; in the 

 protozonite stage this layer is only one cell deep, and within the 

 cephalothorax it is true mesoblast, but in the abdominal area it is 

 mesentoblast. The mesoblast of the cephalothorax splits into a 

 somatic and a splanchnic layer. The mesentoblast of the abdomen 

 extends along its whole length, and is not limited to the caudal lobe ; 

 it splits first into an outer layer of somatic mesoblast and an inner 

 layer of mesentoblast, then the inner layer separates into splanchnic 



