The Jfonthly Mlcroscoplcan 
Journal, January 1, 1869. J 
in some Groujos of Grustaeea. 
43 
consequently a true chorion. This envelope has a micropylar orifice, 
and thus the ovum in its native state consists of — 
1st. A chorion, with its micropyle. 
2nd. A vitellus, composed of 
(a) a clear and transparent protoplasm ; 
(b) nutritive elements, suspended in this protoplasm. 
3rd. A germinal vesicle, which is the nucleus of the primi- 
tive cell. 
4th. A nucleolus, the germinal dot or " spot " of Wagner. 
The ovum of Chondracanthus undergoes entire segmentation of 
the yolk. This process goes on in the usual manner, till there are 
eight distinct segments or globes produced. But at this period 
each of these globes divides immediately, not only into two, but 
into four new globes. The number is thus directly increased from 
eight to thirty-two, and from thirty-two to 128. The planes of 
division all pass through the centre, so that each segment has the 
form of a pyramid, its base being turned towards the surface of 
the ovum, and its apex being at the centre. 
Before the segmentation terminates there is seen to take place 
a separation between the two constituent principles of the vitellus : 
the protoplasm with the nucleus of the segment passes toward the 
periphery of the ovum ; the nutritive elements accumulate in the 
centre, and then the boundary between the different segments dis- 
appears. This process continues until all the nutritive matter is 
accumulated in the centre of the ovum, and there remain at the 
periphery only the transparent cells forming the blastoderm. It 
follows from what we have said: — 
(1.) That the formation of the blastoderm results from the 
division by multiplication of the ovum-cell, and from the separation 
of the vitelline elements from the protoplasm. 
(2.) That each vitelline globe gives rise to a blastodermic cell. 
(3.) That the formation of the blastoderm is not, as is ofkn 
stated, a phenomenon occurring subsequent to segmentation; the 
latter should be regarded as a first phase in the formation of the 
blastoderm. 
2nd Tyjpe. Galigus, Anchorella, Clavella, Lernea Gongiericola, 
and Eudactylina. 
In all these Lerneans the ovum has exactly the same consti- 
tution as that of Chondracanthus ; we have not, however, been able 
to recognize the existence of a micropyle. Total segmentation of 
the yolk does not occur. The first phenomenon which occurs 
in the fecundated ovum consists in the complete separation of the 
protoplasm from the nutritive elements. The protoplasm condenses 
