110 Dr. M. Schultze an tlie Development 



From the forcgoinc: statomcnts, the development of Arenicola 

 has the crreatest similarity with that observed by Mihie-Kdwards 

 ill Terehvlln and Protuln. In these, also, the ejjcp^s are de|)osited 

 without any envelope besides the vitelline membrane in gelatinous 

 massis, in whieh the younc; are developed to a certain ])oiiit. 

 They also obtain an anterior and posterior cirelet of cilia, by 

 means of which they move about in the soft jelly, and do not 

 quit this nntil their more powerful locomotive organs, the bristles, 

 are developed, and the cilia have disapjieared, so that a free 

 swinmiing condition does not occur. Nevertheless there is a 

 difference in the number of bands of cilia, as the last-mentioned 

 forms do not acquire the two fine circles which occur in Arenicola 

 before and behind the broad band. But no great stress can be 

 laid u])on this diflference, as the increased number of ciliary circles 

 appears in this case to be rather a division of the original simple 

 anterior band, and they are all situated upon the same segment, 

 the head. Milne-Edwards supposes that the young animals, 

 after the development of the first cilia upon their surface, creej) 

 out of the vitelline membrane, which is afterwards absorbed. It 

 appears to me more probable, that in Terehella and Prohda, as 

 in Arenicola, the vitelline membrane ])asses into the embryo 

 itself, by furnishing the envelopes of the globules of segmentation, 

 or the future embryonal cells, and that consequently no egg- 

 capsule exists from whieh the embryos must escape. Milne- 

 Edwards did not observe the process of segmentation, and was 

 consequently in uncertainty as to the part taken in it by the 

 vitelline membrane. 



Remak, in his recent investigations upon the development of 

 the Vertebrata, has described the part played by the vitelline 

 membrane, which he calls the eyy-cell-membrane [Eizellenmem- 

 bran), in the segmentation of the egg of the Frog, as consisting 

 in its furnishing envelopes for the segmentary divisions by the 

 agency of constrictions, whieh it acquires simultaneously with 

 the vitelline mass itself. I have confirmed this statement in the 

 eggs of Fetromyzon Planeri, which also undergo a total segmen- 

 tation*. I doubt, however, the propriety of adopting the name 

 of egg-cell-membrane for the membrane immediately enveloping 

 the yelk, as this and no other deserves the name of vitelline 

 membrane. I believe we must regard the vitelline membrane 

 as having the same signification in the eggs of Arenicola as in 

 these eggs. 



In other Branchiferous Annclides, however, the behaviour of 

 the membranes of the c^^ appears to be different. At least in 

 the case of Hermella, Quatrefages asserts that during seginenta- 



* Sec Annals, May 185G, ]). 443. 



