MARINE TELEOSTEAN DEVELOPMENT. 69 



towards the animal pole, takes place. At this pole, always 

 on the lower side of the floating egg, the protoplasm collects, 

 forming a faintly straw-coloured cap in which granules are 

 scattered (Fig. 16). At this part the first segmentation- 

 nucleus may be seen. 



Soon after the appearance of this cap, e.g. in 40 minutes in 

 the whiting, longer in the cod, an indentation begins in the 

 middle which gradually deepens until the cap is bisected 

 (Fig. 17). Thus, instead of one, two segments are now present. 



Fig. 17. Egg of Wliiting. Commencing segmentation. E. E. P. 



In a similar manner the splitting proceeds in each of these, 

 resulting in the formation of four segments, and these again 

 are subdivided by bisecting lines, so that eight segments result. 



This process is continued with considerable, but by no 

 means absolute, regularity. In the colder months during 

 which these changes take place in the cod it is a much slower 

 process than for instance in the whiting, which at a later season 

 develops more rapidly. 



In other words, the rapidity of segmentation is directly 

 dependent on the temperature. Thus the egg of a whiting, 

 observed by Prof. Prince, which showed at 6.40 the first furrow, 

 presented at 9 p.m. the accompanying changes (Fig. 18, a, b, c, 

 d, e) — the last stage (e) having the specks or nuclei in the 

 segments. 



Thus the whole disc, passing from the 8 to the 16, 32 and 

 higher stages, at last forms a finely divided cap (often termed 

 the blastoderm), which is now said to be in the many-celled 

 stage ; this is reached on the second day. The whole process of 

 segmentation thus consists of the dividing up of the protoplasmic 



