The Development of Theridium. 315 



mesiad beneath the ganglia (Fig. 68d) ; they are discontinuous trans- 

 versely and longitudinally. 



In the abdomen each of the eight segments has beneath the ecto- 

 blast {Ed., Fig. G9) a layer of somatic mesoblast {So. Mes.) and one 

 of mesentoblast {Mes. E.) ; and at the base of each limb bud {Ab. 

 5.B) there is a coelom {Coel.) formed, as in the thorax, by secondary 

 separation of the layers ; right and left sacs of the two sides are sepa- 

 rated by the ventral sulcus, but on each side of this groove the meso- 

 blast sacs are longitudinally connected. The first segment, over- 

 looked by so many observers, also has two layers of mesoblast {Ab. 1, 

 Fig. 70), which is separated from that of the hindmost thoracal seg- 

 ment {L. 4) ; it has also its own pair of nerve ganglia {Ab. G. 1, 

 Figs. 63, 66). Within the caudal lobe {Caud., Fig. 69, PI. VI) 

 the mesentoblast is still for the most part one-layered. 



At this stage appears distinctly, and for the first time, a portion 

 of the definitive entoblast. Its cells lie in the abdominal region be- 

 tween the mesoblast and the vitellocytes, are at first smaller than the 

 vitellocytes, but soon increase in size and develop ramifying processes 

 so as to resemble minature vitellocytes. They are to be found from 

 the first abdominal segment {Ent., Fig. YO) posterior to the caudal 

 lobe {Ent., Fig. 69), and would appear to arise disconnectedly from 

 the mesentoblast in its whole extent. At present the entoblast cells 

 occur sparingly and in small groups. Where they are present three 

 layers can be distinguished between the ectoblast and the vitellocytes: 

 somatic mesoblast {So. Mes., Fig. 70), splanchnic mesoblast {Sp. 

 Mes.) and entoblast {Ent.). From the account of this and other 

 stages it will be seen that the vitellocytes take no part in producing 

 the entoblast. 



Still another process is commencing, the production of blood cells. 

 The extraembryonic blastoderm (Fig. 68d) consists of only one 

 layer, ectoblast, for mesoblast is formed only within the embryonic 

 body, and not until later stages does it grow outward from this body. 

 These extraembryonic cells remained quiescent during the gastrula- 

 tion period, later proliferated some of the vitellocytes, and now are 

 giving rise to blood cells. Such cells are marked Bl in Figs. 68d 

 and 6 Be, scattered groups or islands of cells produced by multif>lica- 



