Ova of the Nudibranchiate Mollusca. 387 



structures arc much more opake. The loose membrane surround- 

 ing the body at a later period (fig. 20 v) adheres to the inner sur- 

 face of the shelly and like the other soft textures and the retractor 

 muscle, contains numerous small cells or nuclei. At this period 

 the embryo was never seen to draw the ciliated discs within the 

 shell. 



The ova of the other Nudibranchiate Mollusca examined passed 

 through the same stages of development as those of the Doris 

 bilamellata, and the embryos presented, with some slight modi- 

 fications in size and position, the structures we have described. 

 The embryo of the Doris tuberculata, at the time it leaves the 

 case-membrane, is larger than the others, measuring about 1- 100th 

 of an inch in length and 1-130 in the antero-posterior direction ; 

 that of the Dendronotus arborescens measured 1-165 in length, 

 and 1-250 in the antero-posterior direction ; and that of the Doto 

 coronata about 1-200 in the former and 1-260 in the latter di- 

 rection. The shell of the embryo of the Doris tuberculata is re- 

 latively shorter in length or in the vertical direction than in the 

 other embryos examined, and the parts which protrude beyond 

 the -shell are not only positively but relatively larger. The ciliated 

 discs are especially large, the apex of the foot is narrower and 

 more pointed, and the transparent cells in the base of the foot 

 are relatively smaller. The gullet and stomach are short, and 

 the two large cells at their lateral surfaces are placed near each 

 other at a later period of its development than in the D. bilamel- 

 lata. The cells at the termination of the intestine are more nu- 

 merous and transparent, and occupy a considerable space of the 

 upper part of the right side of the shell, so that the body of the 

 embryo lies more to the left than to the right side of the mesial 

 line of the shell. 



The cilia were not observed on the upper part of the ovum of 

 the Goniodoris Barvicensis and Polycera quadrilineata until the 

 seventh day, or one day later than in that of the D. bilamellata, 

 and the embryos of the Polycera did not begin to leave the spawn 

 until the eighteenth day. The transparent cells in the base of 

 the foot are, from the less opacity of the body, seen at an earlier 

 stage in the embryos of the Pohjcera (fig. 14 a?), and in a great 

 number of these also at this stage, an opake irregular patch, com- 

 posed at least partly of aggregated cells, lay on the surface of the 

 lower end (fig. 14 6). The development of all the ova of the 

 same spawn does not proceed pari passu, but much greater irre- 

 gularities were observed in the ova of the Polycera quadrilineata 

 than in the others ; for in many of these the bipartite division 

 did not even proceed regularly, nor were the cells into which it 

 divided of the same size. That these irregularities were not 

 entirely dependent upon the artificial conditions under which 



