698 PROFESSOR W. C. M‘INTOSH AND MR FE. E. PRINCE ON 
IV. SEGMENTATION. 
At the time segmentation begins (always within a few hours after fertilisation) 
the process of segregation is to a great extent completed, and the germinal disc is 
defined as a thickened patera of clear protoplasm lying upon the yolk in those forms 
whose upper segment is the animal pole, or depending from the yolk in those ova 
with an inferior animal pole, and separated by an intermediate stratum, which differs 
both from the yolk and the germinal matrix. Thus ova of the haddock, fertilised at 
2 p.m. on 23rd March 1886, showed at 8.50 P.M. a uniform prominent mass or cap of 
protoplasm without trace of segmentation. At the margin were numerous protoplasmic 
processes, rising in some cases on the surface of the yolk into globules. On the second 
day the rim of some of the granular spheres projected beyond the dise at the lower pole. 
Whether the separate cells, seen during development, in the perivitelline space are 
due to these projections is unknown. In the cod, again (see Pl. X. fig. 9), the spheres, 
which differ in size, show minute granules. The nuclei of the spheres are not always 
easily seen in the living egg, but with due care can generally be made out. 
Ryper is right in saying that the cleavage does not at first go quite through the dise, 
the contrary being stated by Kiycstey and Conn. The latter authors noticed marked 
amceboid movements at the 4- and 8-celled stages, processes being sent out by the spheres. 
In the early stage of segmentation the Teleostean egg shows external larger spheres 
and internal smaller ones (PI. IX. fig. 8), just as Janostk* found in Crenilabrus and 
Tinea, the internal dividing more quickly than the external. This, likewise, is observed 
in the Elasmobranch ege. 
We have seen that all the features of the fertilised ovum may appear to some extent 
in the unfecundated egg, and though seomentation is usually an indication that fertilisa- 
tion has taken place, it is not infallibly so. OxrtiacuEr found cleavage-lines passing 
across the germ in an unimpregnated egg of the fowl (No. 113),+ and in Teleostean ova 
the dise may break up into segments by an irregular kind of cleavage. Its abnormal 
character is soon revealed, resembling as it does the cleavage of unhealthy and dying 
eggs, the cells always showing great irregularity, and the protoplasm composing them 
assuming a more or less marked opacity or a granular appearance. Both in size and 
shape LerEBouLLer found that these abnormal cleavage-segments differed from the 
normal (No. 93, p. 485). 
The Cortical Protoplasm.—tThe blastodise is formed by the segregation at one pole of 
protoplasm, which, moreover, constitutes a superficial and tenacious layer around the 
vitellus. This layer is itself derived by centrifugal transference from the scattered proto- 
plasm mingled with the general matrix of the yolk, a phenomenon which recalls the 
formation of the periblastula in the crustacean ovum, such as that of Astacus. In this ovum 
* Archiv f. Mikr. Anat., vol. xxiv. 
+ BiscHorr (Ann. d. Sci. Nat., iii. sér., Zool., t. ii.), Hensen (Centralblatt f. die Med. Wiss., 1869), Kipp (Quart. 
Jour, Micr. Sci., xvii., 1877), and others have confirmed OrLLacueEr’s observation in other forms, especially Mammals. 
