The Development of Theritlium. 323 



to the cerebral ganglia (Fig. 87a), where a cleft between them marks 

 the cephalic end of the heart. In comparison with them the cheliceral 

 sacs are small, but little larger than those of the thorax; they are 

 seen on median section in Fig. Ola {Chel. Coel.) postero-lateral to 

 the stomodaeum (Sto.) and they show a cellular thickening of the 

 wall next this; what this thickening may represent I do not know, 

 unless it be a portion of the poison gland. They are continuous with 

 the rostral sacs only along the stomodaeum. Within the abdomen 

 there are separate coelomic cavities for segments one to five inclusive, 

 but with the fusion of the more posterior abdominal segments their 

 mesoblast sacs have fused to compose a pair that extend into the 

 caudal lobe; these may be detected on surface view (Caud., Fig. 82) 

 and more clearly on transverse section (Coel., Fig. 87c). 



With the movement of the caudal lobe and its consequent elevation 

 above the abdominal area {Caud., Figs. 84, 86) it has come to in- 

 clude an axial tube of definitive entoblast. Fig. 87c, M. G., shows 

 this mesenteron on transverse section bounded on either side by meso- 

 blast, and Fig. 91b on longitudinal section. The latter figure illus- 

 trates how the inner end of this tube is continuous with a layer of 

 entoblast (Ent.) next to the yolk (Vit.). The commencement of the 

 mesenteron as a tubular structure is within the caudal lobe, and this 

 tube is anteriorly continuous with a single interrupted entoblastic 

 layer situated at the postero-ventral border of the yolk mass. At no 

 other point in the embryo is there definitive entoblast, but at all places 

 save in a portion of the head region the yolk is bordered by mesoblast ; 

 in the head (Fig. 87a) the yolk is divided anteriorly by the rostral 

 mesoblast sacs into a right and left moiety, each placed between a 

 (more mesial) rostral sac and the (more lateral) thoracal sacs, and 

 anteriorly each yolk moiety comes in contact with ectoblast (see the 

 right side of Fig. 87b). Entoblast appears to be absent in the dorsal 

 abdominal region. 



The reversion of the embryo with the rapid growth of the dorsal 

 margins of thorax and abdomen have produced the heart. This is 

 lettered H in Figs. 82, 84-86, and is a dorso-median tube extending 

 from the cerebral ganglia to the base of the caudal lobe. Interseg- 

 mental boundaries represent the beginning of its vessels, and these 



