made, at Naples in the wintex o/" 1871-72. 85 



Development o/'Aplysia. 



The development of two species of Aplysia was studied in 

 considerable detail as far as the completion of the velum- 

 bearing embryo and its escape from the egg-jelly. Various 

 devices failed of enabling me to observe the later development 

 of this or of several Nudibranchs which were also kept for 

 study. 



The Aplysice were : — a larger species, in which each capsule 

 in the egg-coil contained from thirty to forty embryos ; and a 

 smaller species, in which the number was not more than seven, 

 usually less. The germinal vesicle escapes previously to yelk- 

 cleavage as the " Richtungsblaschen ; " the egg then divides 

 into two larger yellow masses and two smaller pale balls. The 

 pale balls now divide rapidly, and grow over and enclose the 

 larger yellow masses. By a process of multiplication (which 

 I could not satisfy myself was accompanied in Aplysia by 

 invagination, though there were indications of such a mode of 

 growth) the pale cells give rise, not to a single layer of cells 

 enclosing the yellow, but, at the pole whence they started, 

 to a considerable mass or thickness of cells. The deeper 

 of these work themselves in between the two large yellow cells 

 and give rise to the alimentary tract ; the outermost cells form 

 epidermis, nerve, and shell-gland, whilst an intermediate por- 

 tion gives rise to muscles. 



In the two species, however, there is a verycmious difference: 

 for in the larger species the two yellow cells almost as soon as 

 they are enclosed lose their nuclei and definite outline, becoming 

 mere granular masses, which the deep layer of pale cells rapidly 

 invest and attach to themselves in an intimate manner ; whilst 

 in the small species the two yellow cleavage-masses, each with 

 its large bright nucleus, retain their form to the last (that is, 

 as long as I studied the embryos), the deep pale cells (hypo- 

 derm, Darmdriisenblatt) only passing between the two masses, 

 and growing by absorption of the matter which they yielded, 

 as was evident by their gradual thinning out and shrinking, 

 but without being invested or themselves undergoing any for- 

 mative changes. Tlie liver-mass, and perhaps the genital 

 glands, subsequently appear in the position occupied by these 

 two big cells, probably growing out into them, not from them. 



An important fact is the occurrence of cilia on tracts of the 

 pale cells, lying deeply within the segmentation mass ; this I 

 have also seen in the eggs of Pisidium pusilhan. 



The shell-gland is the first organ to appear in Aplysia^ as 

 it is also in the freshwater Lamellibranch Pisidium, and occurs 

 as a groove on the surface, the cells in which take on a special 



