208 BULLETIN OF THE UNITED STATES PISH COMMISSION. 



invaginated cells have wandered a short way only into the egg. Now, if we examine 

 the peripheral cell envelope, we find abundant evidence of cell division over the side 

 of the egg where the embryo proper is soon to be built up. Nests of nuclei, often sur- 

 prisingly large and numerous, are now and then seen in the midst of a spherical mass 

 of yolk either at the surface or just below it. Isolated cells, though few in number, 

 also occur, scattered through the peripheral parts of the yolk at this stage. What 

 is their origin? They can not be referred to invaginate cells, since none of these 

 have yet wandered to remote parts. Furthermore, these cells tend, not to scatter, 

 but to migrate in a body. They may be the descendants of the primary yolk cells or 

 migrants from the peripheral cell envelope, or may originate in both these ways. 



At the time of invagination the egg of Alpheus is very similar to that of the lob- 

 ster in its histological relations. The main difference which is apparent to the eye 

 is in the larger size or greater amount of food yolk in the latter. I have described 

 and figured the invagination stage of Alpheus in some detail in my work on the 

 embryology of this crustacean (94, p. 400, plate xxxi). In this species the primary 

 yolk cells persist and mingle with the wandering cells derived from the invagination. 

 An egg of Alpheus saulcyi in the invagination stage contains about 460 cells, of which 8 

 per cent — exactly 37 were found in two separate eggs — are primary yolk cells (94, p. 

 432, table 1). These yolk cells do not appear to be much more numerous in the larger 

 egg of the lobster (see cuts 21 and 22, showing eggs with 15 and 28 yolk cells respect- 

 ively), but in this animal they degenerate faster than in Alpheus, so that at the 

 invagination period very few are left. On the other hand the occurrence in the lobster at 

 this time of nests of nuclei within the yolk ball, which lies j ust below the surface or some 

 times almost in contact with it, points to migration of cells from the surface after the 

 invagination stage. In any case most, if not all, such cells degenerate and disappear. 



In an older embryo, represented in fig. 254, the pit in the invagination area is 

 considerably enlarged, and below this a solid wedge-shaped column of cells is seen 

 advancing straight down into the yolk or bending slightly toward the hinder end of 

 the embryo. This mass of cells forms what has been called the thoracic-abdominal 

 plate. It here gives rise in part to a mass of cells which migrate into the yolk and 

 eventually form mesodermic and endoderinic structures. I shall call this cell-mass the 

 mesendoderm. Regarding these cells we notice in particular the peculiar association of 

 the cytoplasm with the yolk, the pseudopodia by which they worm their way among 

 the yolk spherules like so many amoeba, the evidence of cell multiplication and of the 

 degeneration or breaking down of cells. 



Signs of cell degeneration are also present in a striking degree over the embryonic 

 area of the egg. The surface cells form a single tier of short prismatic elements 

 loaded with yolk, while beneath them we see a thin cloud of fine chromatin particles. 

 These are mostly the remains of cells which have migrated thither from the mesendo- 

 dermic mass, and possibly in part also of cells which have wandered from the surface. 



The embryo at a little later period has the appearance shown in plate 53. The 

 ingrowing plug of cells has a rounded, somewhat pear-shaped outline when seen from 

 above, the stem end of the pear pointing backward and downward into the yolk. 

 The embryonic area of the egg presents a beautiful mosaic of cells, among which 

 karyokinetic figures are abundant. The dividing planes of these cells are always 

 radial — that is, parallel with a radius drawn from the center to the surface of the 

 egg, but make any angle with a line drawn upon the surface of the egg, such as that 



