The Development of Theridium. 309 



ducing the abdomen is due to a rapid growth between this thin region 

 of the ectoblast and the posterior edge of the germ disc (region of the 

 posterior cumulus), and not to growth cephalad from the posterior end 

 of the germ disc. The abdominal region is therefore extending telo- 

 blastically, by cell multiplication, in its anterior portion. It results 

 that the thin region of the ectoblast (T/i. Ah., Fig. 48) is rapidly 

 separating from the group of vitelloeytes at the posterior end (caudal 

 lobe, Caud.). It will be noted that the thoracal segments develop in 

 situ and almost if not quite simultaneously, teloblastic growth occur- 

 ring only in the abdomen. The extension of the embryo around the 

 yolk, shown in Fig. 47, is due mainly to growth of the abdominal 

 region, dorsal extension of the head lobe being much less in amount. 



The middle cell layer, that between ectoblast and vitelloeytes, is 

 still one cell deep (Fig. 48), and its cells are smaller than those of the 

 ectoblast. This layer within the cephalothoracal region is true meso- 

 blast (Mes., Figs. 50 a-c), for there is at no time any indication that 

 entoblast arises within the cephalothorax ; it is only much later than 

 the stage of reversion that entoblast enters the cephalothorax, and 

 then by growth of the midgut from the abdomen into the posterior 

 part of the thorax. This cephalothoracal mesoblast is segmented, 

 each of its transverse masses confluent with and, indeed, occasioning 

 a protozonite, while it is absent between protozonites ; Fig. 48 shows 

 this condition on median section of the whole embryo, and Fig. 49b 

 on transverse section of one-half of a protozonite. This segmented 

 condition of the mesoblast is a secondary one, for in the preceding 

 stages it showed no such distribution. In the head region (Fig. 50a) 

 there is a layer of mesoblast, as in the thorax, and here also it is 

 segmented, for it shows a division into a more anterior rostral meso- 

 blast (B. Mes.) and a more posterior cheliceral mesoblast (Chcl. 

 Mes.) ; this is important as indicating that in this early stage there 

 are two mesoblastic sacs within the cephalic lobe, one anterior to and 

 distinct from the cheliceral segment. 



In the abdominal region there is a single unsegmented layer of cells 

 beneath the ectol)last, extending from the thoraco-abdominal boundary 

 (Th. Ah., Fig. 50b) not quite to the posterior margin (G. B.) of the 

 germ disc. This layer is mesentoblast, as its later history shows. 



