EMBRYOLOGY OF CLEPSINE. 299 



should by splitting develop a coclom. In this way it is conceiv- 

 able that the schizocccloiis condition niiglit develop from the 

 entero-coelous and gradually lose all trace of its ancestral origin 

 further than is afforded by the derivation of some mesoblastic 

 cells from hypoblast/' Balfour (-rnnr)? pursuing the same hne of 

 thought a little later, remarks : " It might then be supposed that 

 the muscular system of part of the alimentary canal took the 

 place of the primitive muscular system of the body ; so that the 

 whole muscular system of higher animals would be primitively 

 part of the muscular system of the digestive tract." The origin 

 of the mesoderm (Selachii) as " two lateral masses " (Balfour, 

 g.^_i_) lias been confirmed by His {-rrw). The investigations 

 of Kowalevsky upon Sagitta and Amphioxus, above referred to, 

 furnish indubitable evidence of the origin of the mesoderm in 

 two cases. It furnishes such a complete explanation also of the 

 formation of the body-cavity in the Selachians, that one can 

 scarcely doubt that the mesoderm and body-cavity arise in essen- 

 tially the same manner in both cases. Kowalevsky (-rr,V) 

 furthermore states that the origin of mesoderm in the Brachio- 

 pods is similar to that of Sagitta and Amphioxus. Metschnikoff 

 (^^-l) and A. Agassiz (1) have shown that in Echiuoderms the 

 body-cavity arises as lateral diverticula of the entoderm. Putting 

 all these facts together the conclusion first drawn by Lankester 

 appears exceedingly plausible. If this view be correct, it is 

 easy to account for the early appearance of the mesoderm as two 

 mesoblasts, as in Lumbricus, Unio, Pedicellina, Clepsine, and 

 perhaps many other worms and molluscs. It is simply an early 

 expression of its primitive bilateral origin. Before leaving this 

 subject I will call attention to some facts which seem to lend a 

 certain degree of plausibility to the opinion that the mesoderm 

 may at one time arise with the entoderm and at another with the 

 ectoderm. 



In the case of Unio (Rabl) the first division splits the egg 

 into two unequal segments. The larger segment contains all the 

 entoderm, all the mesoderm, and some ectoderm. In Clepsine / 

 the larger segment contains one third of the entoderm, all of ' 

 the mesoderm, and the larger part of the ectoderm. The dif- | 

 ference in these two cases, so far as the mesoderm is concerned, is 

 that in Clepsine the mesoderm goes with the segment that is 

 preeminently ectodermic, while in Unio it goes with the segment 

 that is preeminently entodermic. A'an Beneden (■:rjST) lias 

 found also that the mesoderm goes with the entodermic segment 

 in the rabbit. Selenka has observed the same (Hololhuria,-|-f-i). 

 If the mesoderm in one case is cut oft' with the ectoderm, and in 

 another with the entoderm, it seems not improbable that by a 

 species of cenogenetic heterotopy (Hackel) the mesoderm should 



