52 RICHARD EVANS. 



wall, they are closely packed together and are oval in shape, 

 their long axis being directed at right angles to the surface 

 (PI. 6, fig. 10 e, som.^ and som.^). This rapid differentiation 

 is prophetic of the changes which take place in the somatic 

 wall at an early stage in the development ; that is, of the 

 formation of the myotome and the renal outgrowth. 



In the embryo under consideration the end o derm is 

 present as a layer which completely surrounds the central 

 yolk. There are no cell outlines in it, and the nuclei are not 

 always spherical in shape. The endodermal layer, however, 

 is incomplete at two points, namely, the two portions of the 

 divided blastopore through which the yolk protrudes and 

 spreads itself over the ventral surface as the so-called external 

 yolk. The inner limit of the endoderm is quite distinct from 

 the central yolk, and at the edges of the blastopore it is con- 

 tinuous with the ectoderm. The endodermal nuclei are not 

 situated in the outer layer of the yolk, but in a layer which 

 seems to be new, and entirely different. It is true that it 

 contains spherical masses, presumably stored-up food mate- 

 rial, but there are no compound systems among them, and 

 they stain much less readily. They probably consist of food 

 masses, which the endoderm itself has elaborated at the 

 expense of the central yolk, and stoi'ed up within its own 

 substance. The endodermal layer possesses another charac- 

 teristic which the central yolk completely lacks, namely, a 

 great multitude of small refringent granules, which were not 

 observed in the younger stages already described. The ex- 

 ternal yolk differs from the central yolk in that the masses 

 of food material contained in it are smaller, while the amount 

 of cytoplasm present is larger. It seems that the ectoderm 

 is capable of acting on the external yolk in the same way as 

 the endoderm does on the central yolk, and of building and 

 storing up food masses for future use. This is done chiefly 

 on the ventral surface, where the ectoderm and external 

 yolk are in contact. 



In the embryo under consideration there still remains to be 

 described a most remarkable structure, the nature and signifi- 



