212 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



spond to tlie endoderm sac of the crayfish. In the latter the peculiar cell fragments 

 also occur. 



If one now examines very thin sections under high powers, he finds that the 

 granules and the granulated bodies correspond in general to the structures found in 

 Alpheus. The chromatin grains appear sometimes as naked masses in the yolk, and 

 stain either very intensely or faintly. They are often vesiculated — that is, they appear 

 as hollow shells (fig. 241). Under favorable conditions it is easy to demonstrate the 

 fact that these bodies surround particles of yolk, and occasionally they have a cres- 

 centic shape, when they seem to be enwrapping a yolk spherule (fig. 210, plate 52) 

 (94, p. 127). I have shown that the "secondary mesoderm cells" described in the 

 crayfish by Reichenbach (163) are undoubtedly products of degeneration which are 

 afterwards absorbed in the yolk. In this species the eudodermic cells which are 

 loaded with yolk probably divide by multiple karyokinesis, producing nuclear nests or 

 clusters, some of which in time undergo degeneration. The naked balls of chromatin 

 which are found in these cells are probably formed in situ, though they unquestion- 

 ably shift their position in the egg. 



In a species of Cambarus, which I studied at a stage when five pairs of appendages 

 were present, the endodermal nucleus was surrounded by a thin layer of protoplasm, 

 which worked its way amid the yolk so as to practically surround a pyramidal mass. 

 This strongly recalls the serpentine manner in which the cells creep through the yolk 

 in the egg of the lobster. 



Later, when nine pairs of appendages are represented, the endodermal cells have 

 nearly reached the ectoderm. The yolk within the confines of the ectoderm has an 

 irregular, pyramidal, or radial cleavage. Centrally it blends with a serum-like fluid, 

 in which occasional granules or balls of chromatin are suspended. Small spherical 

 bodies containing a single chromatin ball, or several balls, occur not only in the yolk 

 underneath the ectoderm and in the vicinity of the endodermal nuclei, but also in the 

 central yollc of the endoderm sac at various levels below the endodermal nuclei. This is 

 a point of some interest in connection with the fate of these bodies. They wander not 

 only peripherally but centrally. 1 Earely we meet one which is three or four times the 

 average size, having a small chromatin spherule in its center. These latter become 

 absorbed and gradually disappear (94, p. 128). 



As 1 have already shown, the plasmic vesicles described by Bumpus (30) in the 

 ovarian egg are mesodermic cells in the process of degeneration. (For the origin and 

 history of these bodies see p. 152.) 



Later, according to Bumpus, the plasmic vacuoles are represented by chromatin 

 granules scattered about in the peripheral parts of the yolk. 



In the early cleavage stages Bumpus says that the plasma cells are still represented 

 by chromatin grains, which "are no longer confined to the periphery, however, but 

 have advanced toward the center and formed an indefinite ring" (30). In the speci- 

 mens of eggs in the early cleavage stages w r hich I have studied — stained chiefly in 

 Kleinenberg's hajmatoxylon solution — I have never been able to detect any degenera- 

 tive products whatever. They appear to have been completely absorbed or converted 

 into yolk before this time. 



In a still later period, when the y-shaped embryonic area is differentiated, "the 

 plasma vacuoles," according to Bumpus, "are represented by chromatin nebula}, which 



1 The movement of these bodies is probably due wholly to extraneous mechanical causes 



