698 PROFESSOR W. C. M'INTOSH AND MR E. 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 disc 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 PI. 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. 



Eyder is right in saying that the cleavage does not at first go quite through the disc, 

 the contrary being stated by Kingsley and Conn. The latter authors noticed marked 

 amoeboid 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 Janosik* found in Crenilabrus and 

 Tinea, the internal dividing more quickly than the external. This, likewise, is observed 

 in the Elasmobranch egg. 



We have seen that all the features of the fertilised ovum may appear to some extent 

 in the unfecundated egg, and though segmentation is usually an indication that fertilisa- 

 tion has taken place, it is not infallibly so. Oellacher found cleavage-lines passing 

 across the germ in an unimpregnated egg of the fowl (No. 113),t and in Teleostean ova 

 the disc 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 Lereboullet found that these abnormal cleavage-segments differed from the 

 normal (No. 93, p. 485). 



The Cortical Protoplasm. — The blastodisc 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 



* Archivf. Mikr. Anat., vol. xxiv. 



+ Bischoff (Ann. d. Sci. Nat., iii. se>., ZooL, t. ii.), Hensen (Centralblatt f. die Med. Wiss., 1869), Kidd (Quart. 

 Jour. Micr. Sci., xvii., 1877), and others have confirmed Oellacher's observation in other forms, especially Mammals. 



