110 Dr. M. Schultze on the Deve!opme?it 



From the foregoing statements, the development of Arenicola 

 has the greatest similarity with that observed by Milne-Edwards 

 in Terebella and Frotula. In these, also, the eggs are deposited 

 without any envelope besides the vitelline membrane in gelatinous 

 masses, in which the young are developed to a certain point. 

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

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

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

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

 swimming 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 upon this difference, 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, creep 

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

 appears to me more probable, that in Terebella and Protula, as 

 in Arenicola, the vitelline membrane passes 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 which 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 egg-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, which it acquires simultaneously with 

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

 eggs of Petromyzon 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 Annelides, however, the behaviour of 

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

 the case of Hermellaj Quatrefages asserts that during segmenta- 



* Sec Annals, May 1856, p. 443. 



