126 DEVELOPMENT. 



of the classes formed upon its own type. And so also with the 

 highest organized mollusca ; after their first stage they resemble 

 the simpler orders of their own subkingdom, but not those of 

 any otlier group." 



The molluscan ovum (xix) consists of a colored yolk (vitellus), 

 surrounded b}^ albumen and either enclosed separately by a 

 shell, or several together by a common envelope. On one side 

 of the yolk is a pellucid spot, termed the germinal vesicle, 

 having a spot or nucleus on its surface. This germinal vesicle 

 is a nucleated cell, capable of producing other cells like itself; 

 it is the essential part of the egg, from which the embryo is 

 formed ; but it undergoes no change witliout the influence of the 

 spermatozoa. After impregnation, the germinal vesicle, which 

 then subsides into the centre of the j^olk, divides spontaneously 

 into two ; and these again divide and subdivide into smaller and 

 still smaller globules, each with its pellucid centre or nucleus, 

 until the whole presents a uniform granular appearance {morula 

 stage). The next step is the formation of a ciliated epithelium 

 on the surface of the embryonic mass ; movements in the albumen 

 become perceptible in the vicinity of the cilia., and they increase 

 in strength, until the embryo begins to revolve in the surrounding 

 fluid.* 



Up to this point nearly the same appearances are presented 

 by the eggs of all classes of animals — they manifest, so far, a 

 coitiplete " unity of organization." In the next stage, the devel- 

 opment of an organ, fringed with stronger cilia, and serving 

 both for locomotion and respiration, shows that the embryo is 

 a Tuolluscous animal; and the changes which follow soon point 

 out the particular class to which it belongs. The rudimentary head 

 is early distinguishable by the black eye-specks; and the heart 

 by its pulsations. The digestive and other organs are first 



* According to tlie observations of Professor Loven (on certain bivalve 

 mollusca), the ova are excluded immediately after the inhalation of the 

 spermatozoa, and apparently from their influence ; but impregnation does 

 not take place within the ovary itself. The spermatozoa of Cardium 

 pygmoeum were distinctly seen to penetrate in succession the outer envelopes 

 of the ova, and arrive at the vitellus, when they disappeared. With 

 respect to the " germinal vesicle ;" according to Barry, it first approaches 

 the inner surface of the vitelline membrane, in order to receive the 

 influence of the spermatozoa ; it then retires to the centre of the yolk, 

 and undergoes a series of spontaneous subdivisions. In M. Loven's 

 account it is said to "burst" and partially dissolve, whilst the egg remains 

 in the ovary, and before impregnation ; it then passes to the centre of the 

 yolk, and undergoes the changes described by Barry, along with the yolk, 

 whilst the nucleus of the germinal vesicle, or some body exactly resembling 

 it, is seen occupying a small prominence on the surface of the vitelline 

 membrane, until the metamorphosis of the yolk is completed, when it 

 disappears. It has been named polar globule, and is supposed to be 

 expelled by the micropyle. 



