GEPHALOCHORDA. 5 



layer above this plate before the plate itself is folded over into a 

 closed canal. This peculiarity will be easily understood from an 

 examination of fig. 3 A, B and C. 



The formation of the mesoblastic somites commences, at about 

 the same time as that of the neural canal, as a pair of hollow 

 outgrowths of the walls of the archenteron. These 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 gra- 

 dually extend backwards as wing-like diverticula 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 con- 

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

 — the somatic, and a cavity between the two which was originally 

 continvious 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 mesoblast, and the 

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

 mesoblastic somite long retains its communication 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 longi- 

 tudinal fold (fig. 3 B, ch), which is gradually separated off from before 

 backwards as a rod (fig. 3 C, ch), 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 mesen- 

 teron. 



With the formation of the central nervous system, the mesoblastic 

 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 these are being formed, remains 

 cylindrical, but shortly afterwards it becomes pointed at both ends, 

 and the caudal fin appears. The fine cilia covering the larva also 

 become replaced by long cilia, one to each cell. Tlie mesenteron is 

 still completely closed, but on the right side of the body, at the level 

 of the front end of the mesenteron, the hypoblast and epiblast now 

 grow together, and a perforation becomes formed through their point 



