EXISTENCE BETWEEN BODY-CAVITY AND VASCULAR SYSTEM. 21 



this cavity on the upper side, towards tb^ endoderm, become only 

 one cell thick, they form the splanchnopleure. The opposite wall, 

 the somatopleure, that next the ectoderm, is however several cells 

 thick. 



The anterior wall of each somite fuses with the posterior wall 

 of the preceding somite, and thus septa, comparable to those of the 

 higher worms, are formed, and persist for a short time in embryonic 

 life. Soon, however, the somites fuse with one another, and their 

 cavities become continuous. Then the walls of the two lateral 

 cavities which are thus formed, and at first are only in the ventral 

 face of the embryo, commence to grow round the endoderm. Part 

 of the tissue forming the septa persists as the dorso-ventral muscles. 

 The spaces on each side, growing dorsally and ventrally, fuse, and, 

 by the arrangement of the dorso-ventral muscles two longitudinal 

 septa are formed which divide the common space into a dorsal, 

 ventral', and two lateral sinuses. These are the blood sinuses, 

 which by the development of the connective tissue and muscles 

 become relatively much smaller in the adult than in the embryo. 



Nusbaum further describes and figures the development of the 

 dorsal and ventral vessel, both of which apparently arise as a solid 

 cord of cells, proliferated from the splanchnic layer of the meso- 

 derm, in the middle dorsal and ventral line. They subsequently 

 acquire a lumen, and, separating off, lie in their respective sinuses. 



The same author, in describing the development of the nephridia, 

 points out that in the young embryo. they appear in every somite, 

 even in those which form the posterior sucker, where they subse- 

 quently abort. 



Thus with regard to the origin of the space and the opening 

 into it of the nephridia, the sinuses of the Hirudinea are truly 

 coelomic, the embryological researches of Nusbaum confirming in 

 a most striking way the predictions of Bourne. 



If we turn to the third characteristic of a coelom, that "its 

 lining gives rise to the generative products," the evidence is not 

 quite so satisfactory. The origin of the reproductive cells is 

 probably an example of " precocious segregation." The sexual cells 

 arise from the mesoblasts — the segment cells of Whitmann — which, 

 arising posteriorly, multiply and pass forward till a heap of them 



