I02 



XVII. DEVELOPMENT. 



The observations which have hitherto been made on this subject are of the most fragmentary 

 nature. The "Challenger" specimens of C. dodecalophus, although containing numerous eggs, each 

 enclosed in a vitelline membrane and lying freely in the cavity of the coenoecium, were obviously 

 obtained at a period shortly before the normal developmental season, and nearly all the eggs 

 are unsegmented. Masterman's observations (98, 2, p. 513) have already shewn, however, that 

 some of the eggs are developing; and I have myself found several embryos. 



The preliminary note published by Andersson (03) shews that the embrj-os leave the 

 parent colony in the form of ciliated "planulae". In the absence of evidence to the contrary, 

 it may be presumed that Andersson's statements refer to C dodecalopkits, since the larvae were 

 obtained from material dredged in the Beagle Channel. The date on which the dredging 

 was made was Oct. 30, whereas the "Challenger" material, in which development has hardly 

 commenced, was collected on Jan. i i. 



Mv own observations on the development have been made on C. levinseni and C. 

 gracilis, in both of which the coenoecium contains embryos in various stages of development. 

 My examination of this material has been incomplete, partly owing to the difficulties of staining 

 — which are no doubt the result of imperfect preservation — and partly owing to the fact 

 that I have not feit myself justified in sacrificing more than a small jaortion of the unique 

 material, in each case, in order to study the development. The following statements with regard 

 to this subject may, however, be made : — 



(I) The egg of Cephalodiscus contains a large quantity of yolk. 

 (II) Segmentation is complete, although the later stages are influenced by the presence of yolk. 



(III) The embryo leaves the colony at a comparatively early stage. This has been definitely 

 observed by Andersson, whose results are confirmed — so far as the negative evidence 

 is worth anything — by the absence of later stages in the coenoecium of the specimens 

 at present under consideration. 



(IV) The structure of the oldest embryos observed makes it probable that development is direct, 

 and that there is at no time any transparent, pelagic, larval stage. 



(V) The embryos of the two species observed differ widely from one another in general 



appearance, but agree in essential features. 

 (VI) The body-cavity occurs in a completely segmented form before the time of hatching. 

 (VII) The alimentary canal, in the latest stages observed, has the character of a mass of yolk. 



It is unfortunate that even the oldest embryos do not appear to throw any light on the 

 way in which most of the organs of the adult are developed ; and much remains to be done 

 in the study of the development of the Pterobranchia. 



All the developmental stages which have been observed are still enclosed in the vitelline 

 membrane. C. gracilis and C. levinseni have a simple spherical or ovoid vitelline membrane, 

 which is not prolonged into the narrow stalk found in C. dodecaloplms. 



