524 MOLLUSCOUS ANIMALS GENERALLY. 



num) seems to be as follows : — Of tliose 500 or 600 egg-like bodies, 

 only a small part are true ova, tbe remainder being merely yolk- 

 spherules, which are destined to serve for the nutrition of the 

 embryoes. The distinction between them manifests itself at a 

 very early period, even in the first segmentation ; for while the 

 yolk-spherules divide into two equal hemispheres (Fig. 271, b), 

 the real ova divide into a larger and a smaller segment (d) ; in 

 the cleft between these are seen the minute " directive vesicles," 

 which appear to be always double or even triple, although, from 

 being seen " end on," only one may be visible ; and near these 

 is generally to be seen a clear space in each segment. The dif- 

 ference is still more strongly marked in the subsequent divisions; 

 for whilst the cleavage of the yolk-spherules goes on irregularly, 

 80 as to divide each into from 14 to 20 segments having no defi- 

 niteness of arrangement (c, e, f, g), that of the ova takes place 

 in such a manner as to mark out the distinction already alluded 

 to between the cephalic and the visceral portions of the mass (h) ; 

 and the evolution of the former into distinct organs very speedily 

 commences. In the first instance, a narrow transparent border 

 is seen around the whole embryonic mass, which is broader at 

 the cephalic portion (i) ; next, this border is fringed with a short 

 cilia, and the cephalic extension into two lobes begins to show 

 itself; and then between the lobes a large mouth is formed, 

 opening through a short, wide cesophagus, the interior of which 

 is ciliated, into the visceral cavity, occupied as yet only by the 

 yolk particles originally belonging to the ovum (k). Whilst 

 these developmental changes are taking place in the embryo, the 

 whole aggregate of segments formed by the subdivision of the 

 yolk-spherules coalesces into one mass, as shown at a. Fig. 272 ; 

 and the embryoes are often, in the first instance, so completely 

 buried within this, as only to be discoverable by tearing its por- 

 tions asunder; but' some of them may commonly be found upon 

 its exterior ; and those contained in one capsule very commonly 

 exhibit the diflferent stages of development represented in Fig. 

 271, H-K. After a short time, however, it becomes apparent 

 that the most advanced embryoes are beginning to sioallow the 

 yolk-segments of the conglomerate mass ; and capsules will 

 not unfrequently be met with, in which embryoes of various 

 sizes, as a, b, c, d, e (Fig. 272, a), are projecting from their sur- 

 face, their diflerence in size not being accompanied by advance 

 in development, but merely depending upon the amount of this 

 "supplemental" yolk which the individuals have respectively 

 gulped down. For during the time in which they are engaged 

 in appropriating this additional supply of nutriment, although 

 they increase in size, yet they scarcely exhibit any other change ; 

 so that the large embryo. Fig. 272, e, is not apparently more ad- 

 vanced as regards the formation of its organs, than the small 

 embryo. Fig. 271, k. So soon as this operation has been com- 

 pleted, however, and the embryo has attained its full bulk, the 



