938 



MOLLUSCA AND BKACHIOPODA 



this border is fringed with 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 oesophagus, 

 the interior of which is ciliated, into the visceral cavity, occu- 

 pied 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 yolk-cleavage of the 

 infertile eggs coalesces into one mass, as shown at A, fig. 713 ; and 

 the embryos are often, in the first instance, so completely buried 

 within this as only to be discoverable by tearing its portions asunder ; 

 but some of them may commonly be found upon its exterior, and 

 those contained in one capsule very commonly exhibit the different 



FIG. 713. Later stages of embryonic development of Purpura lapillus. 

 A, conglomerate mass of vitelline segments, to which were attached the 

 embryos a, 6, c, d, e. B, full-sized embryo in more advanced stage of 

 development. 



stages of development represented in fig. 712, H-K. After a short 

 time, however, it becomes apparent that the most advanced embryos 

 are beginning to swallow the yolk segments of the conglomerate mass, 

 and capsules will not unfrequently be met with in which embryos 

 of various sizes, as a, b, c, d, e (fig. 713, A), are projecting from its 

 surface, their difference of size not being accompanied by advance in 

 development, but merely depending upon the amount of this * supple- 

 mental' yolk which the embryos 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. 713, e, is not apparently more advanced, as regards the formation 

 of its organs, than the small embryo, fig. 712, K. So soon as this 

 operation has been completed, however, and the embryo has attained 

 its full bulk, the evolution of .its organs takes place very rapidly ; the 



