324 LANGENBECK. [VoL. XIV. 
a plug of mesoderm cells arises at any one point, and that this 
stage had escaped my notice, but rather that in Microdeutopus 
the mesoderm is formed at many points in the ventral plate. 
I have a section of a cell dividing obliquely inwards in the 
anterior end of the head region, and Fig. 51 shows another 
case in which there are two cells in the aster stage. The 
equatorial plate of each spindle makes an acute angle with the 
tangent to the surface of the egg at that point. Had divided, 
the greater part of one daughter cell would have lain in the 
second layer. In Fig. 52 we see a cell () which may have 
been derived from a cell like %, or it may be one of the cells of 
the outer layer drawn under the surface; from the appearance 
of the nucleus I am rather inclined to the latter view. I never 
have seen spindles whose axes were parallel to the radius of 
the egg in cells at the surface, although I have seen radially 
directed spindles in the second layer (Fig. 50). Cells which 
appear as though they were drawing in or had arisen by oblique 
division were found at any point on the ventral plate. I there- 
fore conclude that part of the mesoderm in Microdeutopus is 
derived from the ventral plate. 
When the dorsal pole of the egg has been completely over- 
grown by the ventral plate the ventral portion of the blasto- 
derm, posterior to the head region, is composed of two layers 
of cells. With the digestion of the yolk by the entoderm 
cells, which begins during the third day, all the cells of the 
embryo rapidly increase in number. Cell boundaries are en- 
tirely lost at this stage, the protoplasm of the cells fusing and 
making it impossible to distinguish where one layer ends 
and the other begins. Since the nuclei of both ectoderm and 
mesoderm appear exactly alike, I could not tell whether the 
ectoderm was only one layer deep and the mesoderm many 
layered, or whether the ectoderm consisted of more than one 
layer. Towards the end of the third day numerous spindles 
are found in all the layers making any angle with the tangent 
to the surface. In sections of the fourth day (Figs. 59-64) the 
muscle cells show their characteristic striations. 
Bergh (93), in his paper on Gammarus, suggests that the 
cells of the muscle plates are derived from teloblasts, as he 
