Development of the Food-Fishes. 447 



lucent and tense appearance. Several large enucleate cells 

 often occur near the centre of the yolk at this early stage, but 

 then- significance and fate are not known. 



Formation of the Blastodisc. 



At the lower pole of the yolk-globe the film of pale ochre- 

 tinted protoplasm increases in thickness, and the entire surface 

 of the vitellas appears corrugated. These ridges are, how- 

 ever, very faintly indicated, and they mark meridional areas 

 of transference, along which much of the cortical protoplasm 

 passes to the germinal pole. 



Segregation of protoplasm probably continues during the 

 whole process of cleavage ; but it is most apparent during the 

 first hour after fertilization, when the disc is being formed, as 

 a plano-convex cap of a faint- straw-tint, in which granules 

 sparsely occur and one or more larger vesicles appear. The 

 disc increases not only by peripheral, but also by subgerrainal 

 transference, as is shown by the fact that vesicles and gran- 

 ules may be distinguished, situated partly in the disc and 

 partly in the underlying matrix. Viewed from above, the 

 disc is almost perfectly circular, and has the form of an 

 inverted plaque depending from the yolk, the food-yolk being 

 thus uppermost, in contrast with the Amphibian ovum, in which 

 the animal pole is uppermost and the large food-yolk cells 

 occupy the lower pole. The vitellus, with its germinal pellicle, 

 revolves freely within the capsule, and the embryonic area 

 can thus maintain its ventral position when the ovum is 

 turned over. 



Segmentation. 



A central cavity soon appears below the blastodisc, by 

 which it is lifted away from the yolk, except at the periphery, 

 where the continuity of the disc and the periblast is never 

 broken. This dehiscence was noted in 2\ (jurnardus while 

 the first cleavage was in progress, and in other Teleosteans a 

 similar cavity has been noticed at an early stage ; but it is 

 not usually regarded as representing the true segmentation 

 cavity, the latter being recognized only when the later multi- 

 celled stage is reached. The first cleavage is incomplete, i. e. 

 the disc is not sharply separated from the periblast, and the 

 two blastomeres are confluent below. The second cleavage 

 cuts the first furrow at right angles, and four blastomeres result. 

 Asymmetry is very frequent at the first cleavage, and the two 

 cells show great disparity in size. Doubtless the phenomenon 

 is due chiefly to unequal transference of the ditt'used proto- 

 plasm of the yolk ; but the form and size of the ceils are 



