The Development of Theridium. 345 



they consist of movements of cells within the yolk, but they aro 

 blastodermic and not vitellar. 



The early blastoderm consists of a peripheral layer of cells 

 equally numerous at all points of its surface. The ventral germ disc 

 became established by (1) more rapid multiplication of cells at that 

 region, and (2) to less extent by migration of cells along the periphery 

 toward that jjole. When the germ disc is forming there appear for 

 the first time cell membranes, while before this period the only cell 

 membrane was the vitelline membrane, and the cytoplasm represented 

 one great reticular syncytium. 



During gastrulation there are two movements. First, a sinking of 

 cells below the blastoderm in the region of the two cumuli and along 

 the margins of the germ disc, this due to change in position of spin- 

 dles at those regions. And, second, an active autonomous wandering 

 of the cells (vitellocytes) so invaginated partly into the yolk, but 

 mainly upon it. It is .noteworthy that the wandering of these cells 

 does not carry them into the extraembryonic area, therefore some" 

 chemotactic influence must hold them within the germinal area. The 

 movement of the vitellocytes is amoeboid and, to some extent, inde- 

 pendent of blastodermic changes. The mesoblastic and mesento- 

 blastic elements are again produced by vertical mitoses, but their 

 cells remain parts of the blastoderm. 



The elongation of the germ disc is due to rapidly repeated 

 mitoses in the region just anterior to the caudal lobe, and this is 

 what occasions the dislocation of that lobe around the surface of the 



egg. 



Coincident with this lengthening of the germ disc arise the pro- 

 tozonites by a segmental rearrangement of the previously continuous 

 mesoblast, the latter dividing into successive transverse masses. 

 This is not a result of the lengthening of the embryo, for the cephalo- 

 thoracal protozonites appear after their region has attained its full 

 length. It is rather an autonomous movement of the mesoblast- itself, 

 a dehiscence of its cells at serially repeated points. Neither ectoblast 

 nor yolk cells share in this process. This movement seems to take 

 place by mesoblast cells at certain points passing beneath their com- 

 rades to form the splanchnic layer ; certain cells at the anterior border 



