Dr. W. B. Carpenter on the Development of Purpura lapillus. 1 7 



into from 14 to 20 segments having no definiteness of arrange- 

 ment (c, E, F, g), that of the ova takes place in such a manner as 

 to mark out the distinction, first noticed by Vogt (in his memoir 

 on the development of Actaon), between the cephalic and the vis- 

 ceral portions of the mass (h) ; and the evolution of the former 

 into distinct organs very speedily commences. In the first in- 

 stance, a narrow transparent border is seen around the whole em- 

 bryonic mass, which is broader at the cephalic portion (i) ; next, 

 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 oesopha- 

 gus, the interior of which is ciliated, into the visceral cavity, oc- 

 cupied as yet only by the yolk-segments 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. 2 ; and the embryos are often, in the first instance. 



Fig. 2. 



Later stages of embryonic development of Purpura lapillus : a, conglomerate mass of 

 ntelline segments, to which were attached the embryos, a, h, c, d, e; b, full-sized 

 embryo, in more advanced stage of development. 



SO completely buried within this, as only to be discoverable by 

 tearing its portions asunder ; but some of them may generally be 

 found upon its exterior ; and those contained in one capsule very 

 commonly exhibit the different stages of development represented 

 in fig. 1, 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. 2, a), are projecting from its surface, their dif- 

 Ann.^^ Mag. N. Hist. Ser.'2. Vol.xx. 2 



