^8 DEVELOPMENT OF HETERODONTUS (CESTRACION) PHILLIPI, 



longer axis corresponding with the future long axis of the body. 

 At its posterior end appears a crescentic dark area which has very 

 much the appearance of a cleft ] massing right through the blasto- 

 derm, but which sections prove to be a cavity, the segmentation 

 cavity, covered over b}'- a thin transparent roof. As the blasto- 

 derm extends, this dark area becomes less strongly mai-ked and 

 graduall}- disappears. 



The yolk is covered with a thin investment which is perfectly 

 continuous with the non-nucleated jirotoplasmic network of the 

 substance of the yolk, of which it is to be looked upon as a 

 specially modified part. 



The light yellow band referred to above extends more rapidly 

 than the blastoderm, and soon forms a broad zone around the 

 latter. As it extends its boundaries become more and more 

 indistinct. This is due to the spreading out of the bed of fine- 

 grained parablastic substance on which the blastoderm lies. A 

 number of small rounded spots, which appear scattered over it, 

 are found on the examination of sections to be produced by the 

 development of rounded spaces or vacuoles. 



The earliest specimen of which satisfactory sections were 

 obtained (Fig. l)is one in which the fine-grained bed of yolk extends 

 beyond the edge of the blastoderm to rather more than half of 

 the diameter of the latter. In this stage the blastoderm consists 

 of a lenticular mass of chiefly rounded cells, resting directly on 

 the fine-grained substance in the greater part of its extent, but 

 becoming separated from the latter towards the posterior end by 

 a small segmentation cavity. The most superficial layer of cells 

 are closely packed together : they are irregular in size and shape, 

 but form a tolerably definite layer. In the deeper strata the cells 

 are more loosely arranged, with intercellular spaces. In these, as in 

 the segmentation cavity, with which they are more or less directl}' 

 continuous, there are irregular masses and strands of a finely 

 granular material, which is strongly coloured by staining agents; 

 frequently this matter adheres to the surface of the cells or the 

 wall of the segmentation cavity so as to form a distinct invest- 

 ment : from its apj^earance and mode of occurrence this material 



