82 MCMURRICH. [Vol. XI. 



of the ectoderm becomes as well-marked as in Fig. 17. The 

 dorsal view (Fig. 18) presents a very different appearance from 

 the ventral. In the first place, the ectoderm cells are much 

 more widely separated from one another, and secondly, the 

 mesoderm no longer forms a complete ring, but a break in its 

 continuity has appeared in the mid-dorsal line, and, further- 

 more, it will be seen that the band thus formed tapers rather 

 suddenly toward either extremity. These appearances can 

 only be explained satisfactorily by supposing that a migration 

 of the ectodermal and mesodermal cells is taking place towards 

 the ventral surface, or to express it slightly differently, that all 

 the mesoderm cells and part of the ectoderm is concentrating 

 upon the ventral surface to form the embryo. 



This concentration has become more marked in the stage 

 represented in Fig. 19. The preparation is figured from the 

 ventral surface, and it is seen that the mesoderm band has be- 

 come broader along the ventral mid-line, or, at any rate, that it 

 is composed of a greater number of cells in that region than it 

 was in the preceding stage, and at the same time the bands 

 have thinned out considerably at the sides where before they 

 were broadest. The liver endoderm (/. en) is still visible, 

 though less prominent than before, and the vitellophags have 

 divided, as may be seen from their marked approximation in 

 pairs. The ectodermal cells on the ventral surface have now 

 come into close apposition so as to form a plate lying in front 

 of the mesoderm band, the front edge of the plate being some- 

 what deeply notched in the mid-line. The cells composing it 

 are well defined, and in the preparations are distinctly sepa- 

 rated from each other, possessing no protoplasmic processes as 

 in preceding stages, and sections show that they now give off 

 no processes into the yolk, which seems to be entirely destitute 

 of protoplasm and forms a central purely nutritive mass, upon 

 the surface of which the cells lie. So far as the ectoderm of 

 the plate and the mesoderm are concerned the syncytial condi- 

 tion no longer exists, though apparently the dorsal ectodermal 

 and the vitellophag cells still are united through the interven- 

 tion of the peripheral protoplasm. As regards the ectodermal 

 cells, they are rather widely scattered, and their protoplasm is 



