The Development of Theridium. 319 



The mesoblast of the cephalic region shows the same general dis- 

 position as in the preceding stage (rostral mesoblast, R. Mes., and 

 cheliceral, Chel. Mes., Figs. 75b-76c, 80a-d). The cheliceral meso- 

 blast has grown laterad to some degree and also to some extent 

 beneath the cheliceral ganglia (Fig. 76c), but is still separated from 

 the stomodaeum. The thoracal mesoblastic sacs have grown little 

 save that^ owing to the widening of the ventral sulcus, those of the 

 right side have separated further from those of the left, and that 

 those of the same side have come to meet each other longitudinally. 

 Each abdominal appendage has a mesoblast sac with coelom {Ah. 5.b, 

 Fig. 77) and so does the caudal lobe (Caud., Figs. 75a, 80e). 



The entoblast in the stage of 97 hours has increased and is arranged 

 in scattered groups of cells in the region of the abdominal appendages 

 {Ent., Fig. 77) as well as in the caudal lobe (Ent., Fig. 75a) ; its 

 disposition indicates continuing formation from the mesentoblast. 

 In the stage of 108 hours it forms a nearly continuous sheet beneath 

 the caudal lobe and the segments immediately anterior to this, seen 

 best on median section {Ent., Fig. 80e) ; its nuclei are smaller and 

 flatter than those of the splanchnic mesoblast {8 p. Mes.). 



The extraembryonic blastoderm is continuing the process of pro- 

 liferating blood cells, and as these cells separate from the blastoderm 

 they migTate upon the yolk to get into the embryonic body. Figs. 75a 

 and 75b show some blood cells {Bl.) still extraembryonic, and other 

 larger ones that have moved beneath the caudal lobe. Fig. 80a ex- 

 hibits a group of them developing just anterior to the head lobe, 

 and Figs. 76a and 80c just lateral to it. Those that have reached the 

 embryonic body lie for the most part between it and the yolk (or 

 vitellocytes), but occasionally some occur within the mesoblast or 

 even the coelom. There is no indication that any of the blood cells 

 are mesoblastic or embryonic in orgin ; on the contrary, the centers of 

 formation lie exclusively in the extraembryonic blastoderm, and the 

 latter has now ceased to produce vitellocytes and is forming blood 

 cells only. 



The nerve ganglia of the stage of 97 hours are little diiferent from- 

 those of the preceding stage, but at 108 hours (Figs. 78, 79) they 

 are hardly distinguishable on surface views. The cheliceral ganglia 



