MESOBLASTIC SOMITES. 



outgrowths, which are shewn in surface view in fig. 2 B and D, 

 so, and in section in fig. 3 B and C, so, arise near the front end 

 of the body and gradually extend backwards as wing-like diver- 

 ticula of the archenteric cavity. As they grow backwards their 

 dorsal part becomes divided by transverse constrictions into 

 cubical bodies (fig. 2 D and E), which, with the exception of the 

 foremost, soon cease to open into what may now be called the 

 mesenteron, and form the mesoblastic somites. Each mesoblastic 

 somite, after its separation from the mesenteron, is constituted 

 of two layers, an inner one the splanchnic and an outer the 

 somatic, and a cavity between the two which was originally con- 

 tinuous with the cavity of the mesenteron. Eventually the 

 dorsal parts of the outgrowths become separated from the 

 ventral, and form the muscle-plates, while their cavities 

 atrophy. The cavity of the ventral part, which is not divided 

 into separate sections by the above described constrictions, 

 remains as the true body cavity. The ventral part of the inner 

 layer of the mesoblastic outgrowths gives rise to the muscular 

 and connective tissue layers of the alimentary tract, and the 

 dorsal part to a section of the voluntary muscular system. The 

 ventral part of the outer layer gives rise to the somatic meso- 

 blast, and the dorsal to a section of the voluntary muscular 

 system. The anterior mesoblastic somite long retains its com- 

 munication with the mesenteron, and was described by Max 

 Schultze, and also at first by Kowalevsky, as a glandular organ. 

 While the mesoblastic somites are becoming formed the dorsal 

 wall of the mesenteron develops a median longitudinal fold 

 (fig. 3 B, c/i), which is gradually separated off from before back- 

 wards as a rod (fig. 3 C, c/i), underlying the central nervous system. 

 This rod is the notochord. After the separation of those 

 parts the remainder of the hypoblast forms the wall of the 

 mesenteron. 



With the formation of the central nervous system, the meso- 

 blastic somites, the notochord, and the alimentary tract the 

 main systems of organs are established, and it merely remains 

 briefly to describe the general changes of form which accompany 

 the growth of the larva into the adult. By the time the larva 

 is but twenty-four hours old there are formed about seventeen 

 mesoblastic somites. The body, during the period in which 



