150 MARINE BIOLOGICAL LABORATORY. 



former case, multipolar delamination ; but owing to the 

 large amount of yolk, the delamination begins first in 

 the region of the micromeres, — each cell of which 

 delaminates into an inner and an outer cell, — and this 

 process progresses slowly over the surface of the Qgg, 

 each cell, both micromere and macromere, delaminating 

 just before it joins the edge of the ever-widening blasto- 

 derm. The outer cells of the blastoderm form a some- 

 what flattened columnar layer, under which lie the 

 nuclei and protoplasm of the inner cells. To the inner 

 cells would belong properly all of the yolk of the 

 pyramidal cells; but as a matter of fact, all traces of 

 cell-outlines are completely lost in the yolk at this time, 

 so that the blastoderm, with its two layers, lies at the 

 surface of a yolk-mass. Soon after this there can be 

 seen at the surface of the embryo a round opaque spot, 

 which I shall call the primitive cumulus. The ectoderm 

 is thickened in this region, and in the centre of the 

 thickening lies the triangular invagination of the stomo- 

 daeum, and around the latter appear the first traces of 

 the mesoblast. 



There next appear at the surface of the embryo 

 other opaque areas, which are the beginnings of the 

 brain, the ventral ganglia, and four pairs of appen- 

 dages ; — viz. the first, fourth, fifth, and sixth. The 

 embryo still retains its spherical shape. 



The next stage is shown in ventral view by Fig. 3, 

 By reference to the figure the more important structures 

 can be seen. The proboscis projects forward ; on each 

 side of this are the mandibles. On each side of the 

 ventral median line lie five pairs of appendages, and to 

 the sides of the last three are three pairs of walking- 



