OBSERVATIONS ON DEVELOPMENT OF COMMON TROUT. 115 



masses, of which those nearest to the yolk are largest, the 

 fissures being distributed chiefly over the superficial part of 

 the blastoderm — there is a uniformly granular substance to 

 be noticed around the blastoderm, which, although connected 

 with its (the blastoderm^s) peripheral edge, forms no integral 

 part of the blastoderm itself, and does not participate directly 

 in the process of segmentation. It is this substance and its 

 changes which I am going to describe in the present paper. 

 Let us then commence with the examination of a thin sec- 

 tion through the ovum of a trout of a very early stage of 

 development. 



[Note. — The ova which formed the material for the present 

 research developed somewhat slowly, for only after nine 

 days' incubation did the subgerminal cavity make its 

 appeai'ance.] 



A vertical section through the blastoderm of the third day, 

 which when viewed from the surface has the appearance of 

 being divided into about twelve or sixteen elements, presents 

 the following features : — The blastoderm is of the shape of a 

 plano-convex disc, the free surface being plane or rathervery 

 slightly depressed, the lower, i. e. that resting on the surface of 

 the yolk, being convex; the largest vertical diameter amount- 

 ing to about 0'4 mm. The external margin of the blastoderm 

 does not rest on the yolk, but is overhanging, as it were, like 

 a lip, owing to the presence of a relatively deep groove (see 

 fig. 1). A section comprising the largest horizontal diameter 

 — which is about 1*15 mm. — would therefore cut the blasto- 

 derm in the external half of its largest vertical diameter, 

 as is shown in fig. 1 ; the blastoderm is divided into a number 

 of large elements, which are separated from each other by 

 more or less distinct fissures. These latter, which in my 

 specimens hardened with thin chromic acid, present the 

 appearance of thin septa, are seen to penetrate only to a 

 certain depth, the deepest mass of the blastoderm being free 

 from them, 2. e. being as yet undivided. This corresponds to 

 what 1 have figured in my first paper (1. c, fig. 9), and what is 

 termed by Oellacherio (1. c, p. 23) " basale masse." The 

 blastoderm appears as a granular mass ; the granules, however, 

 are not uniformly distributed through it, for in the depth 

 they are much larger than in the superficial parts, and they 

 gradually shade oflf into those contained in the superficial parts 

 of the yolk. Nevertheless, the boundary by which the lower 

 surface of the blastoderm is separated from the yolk of the 

 saucer-like depression on which the blastoderm rests, is 

 tolerably sharp (see fig. 1); the yolk of the saucer-like 

 depression contains, besides large oil drops — occasionally. 



