312 Thos. H. Montgomery. 



region from the jDcdipalpal to the fourth ambulatory segment. In 

 such ganglionic thickenings {Gang., Fig. 62) the ectoblast is two- 

 layered, while in the ventral sulcus {Sul. v.) between them it has 

 become thinner; therefore, cells have probably moved away from 

 that ventral midline to aid in the production of the ganglia. Such 

 ganglia cannot yet be distinctly seen upon surface views, and those 

 of the chelicera are not yet differentiated. 



In the head lobe the posterior cheliceral mesoblast (CJiel. Mes., 

 Fig. 60b) is distinct from the more anterior rostral (R. Mes., Fig. 

 60a). A slight elevation on the surface of the head lobe seems to 

 indicate the first appearance of the cerebral ridges. In the thorax 

 each appendage has its mesoblast sac, but these sacs extend neither 

 mesial nor lateral of the appendages (Fig. 62, transverse section). 



The abdomen where it is segmented exhibits its mesentoblast in 

 segmented two-layered masses, but in the caudal lobe in a single 

 layer. Each segment has a right and left mesentoblastic mass sepa- 

 rated in the midline from each other (Fig. 61). Where this cell 

 mass is two layers deep the outer layer is somatic mesoblast. {So. 

 Mes.), while the inner is still mesentoblast {Mes. E.). At various 

 points in the median axis of the abdomen are groups of small cells 

 {G. 0. ?, Fig. 61), which had been remarked in the preceding stage. 



Fig. 61 shows that vitellocytes are still forming from the extraem- 

 bryonic blastoderm. 



7. Stage of the Early Ahdominal Appendages (86 Hours). 

 The external characteristics of embryos of this period are shown 

 in Figs. 63-66, PI. V. The caudal lobe {Caud.) has reached the 

 head lobe, the ventral sulcus {Sid. v.) is widening and extends pos- 

 teriorly to the seventh abdominal segment (Fig. QQ), which is one 

 factor in the lateral expansion of the body, and in consequence the 

 extraembryonic area has decreased in amount (the stippling repre- 

 sents the nuclei of this area in their actual number). The lateral 

 view. Fig. 63, shows how the embryonic region has encroached upon 

 the extraembryonic as compared Avith the preceding stage, Fig. 57. 

 Of the cephalothoracal appendages the cheliceral {Chel., Figs 63, 

 65) are the shortest, while the others {L. 1-L. i) have become three- 



