EMBRYOLOGY OF CLEPSINE. 305 



{e) The Neiinila of Clepsine compared with that of Vertebrates. 



The germ-bands in Clepsine, their epibolic growth, and final 

 conjunction at the median neural line, are so remarkably 

 similar to the embryonic rim and the process of neurulation in 

 Vertebrates, as to indicate a fundamental relationship. This 

 similarity has already been noticed by Semper (153) and 

 Hatschek (67), and adduced as an argument in favour of a 

 geneological relationship between the vertebrates and the inverte- 

 brates. Of the justice of the comparison I am thoroughly 

 convinced, and 1 propose here to add some considerations in 

 its favour which have hitherto passed unnoticed. I believe I 

 have already made clear the manner in which the ectoderm 

 with its marginal bands incloses the blastomeres. If my account 

 of the prrecession (Eauber) of the germ-bands is correct, it is 

 evident that Hatschek( y) has a very incorrect notion of the 

 cause for their closing first at the cephalic ends. Hatschek 

 attributes this to a more rapid development of the mesoderm at 

 this end of the bands. I_have shown that the mesoderm 

 develops from the posterior ends of the bands. The muTtiplica- 

 tioh" of meJoderraic'' elements in the fore ends has nothing 

 whatever to do with the early conjunction at this point. 



In the case of Lumbricus, as Kowalevsky and Hatschek have 

 stated, the ])rfecession is more rapid at the hind end, just the 

 inverse of what takes place in Clepsine. The same is true in the 

 frog, as Eusconi (144) long ago pointed out, and in Accipenser 

 (VVrr) • 111 both these cases, however, the approach of the two 

 halves of the embryonic band is at first more rapid near the 

 middle (figs. 7 and 8, g b). Vrhether the union of the germ- 

 bands, or two halves of the embryonic rim, takes place earlier 

 at the fore end than the hind end, or the inverse, is a matter of 

 secondary importance. The fact that such a union does take 

 place, and that the leading features of the prtecession in each 

 case are fundamentally alike, is the point of central interest. 



Professor His has given an excellent account of the manner 

 in which the fish-embryo lengthens backward by the apposition 

 of the two halves of the embryonic rim. That his words give a 

 complete picture of what happens in Clepsine is good evidence 

 that the process described is one and the same in both cases. 



"Man kann den Vorgang veranschaulichen, wenn man eiuen 

 zum King geschlossenen Gummischlauch an eiuer Stelle so 

 einbiegt, dass er eine dem Centrum zustrebende Schleife bildet. 

 Bringt man beide Schleifen-schenkel zur Beriihrung und 

 verliingert sie mehr und mehr, so wird der Eing immer kleiner 

 und schliesslich geht or in der Bildung des zwei-theiligen 



