336 MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



PRODnCTS OF CELL DEGENERATION. 



At the beginning of the egg-nauplias period, when numerous wandering cells have passed 

 forward and joined the inner surface of the embryonic ectoderm, certain new bodies begin to appear 

 in great numbers. They vary in size from small refringent particles to bodies nearly as large 

 as ordinary nuclei. The latter stain deeply and nearly unitbrmly, but with high powers it is 

 often possible to demonstrate a clearer zone about them, which might be mistaken for a layer of 

 cell protoplasm. How do these bodies, the so called " secondary mesoderm cells," originate and 

 what is their function? As to their origin there can be no doubt whatever. They arise by a 

 process of degeneration from the embryonic cells or nuclei, chiefly from those wandering cells just 

 described. Many of the latter may be seen to be swollen out and their chromatin divided into 

 coarse grains and balls of various sizes. The wall of the cell breaks down and thus sets the chro- 

 matin granules free, or, more correctly, the products of the degenerating chromatin. 



These degenerating bodies are most marked in the fully developed egg-nauplius, where there 

 is a large accumulation of them around the oesophagus and at the bases of the rudimentary 

 appendages. After this stage they generally disappear from these regions. Somewhat later, 

 however, when there is a well developed nervous system and six pairs of post-naupliar appen- 

 dages, a patch of ectoderm cells on the surface of the egg opposite the embryo proper becomes 

 noticeable. It reminds one of a median unpaired "dorsal organ." A slight invagination appar- 

 ently takes place at this point, but at any rate a number of cells pass into the surrounding yolk, 

 and these give rise in the way described, to a swarm of minute particles of chromatin products. 



Before any pigment is deposited in the eyes, it is easy to demonstrate the presence of blood 

 corpuscles in the stream of plasma which bathes the nervous system. They have the adult 

 characteristics, that is, they possess a deeply staining nucleus and a clear irregular body. In the 

 nauplius Stage, moreover, some of the larger "secondary mesoderm cells" have a similar appear- 

 ance, but there is no evidence that they ever become blood cells. Mr. Herrick's study of these 

 bodies has shown that Reichenbach's views on the function of secondary mesoderm cells of Astacus 

 are probably erroneous. According to this naturalist they arise from the nuclei of the endoderm 

 cells, forming the ventral wall of the primitive stomach, and are converted into mesoderm. 



THE GERMINAL LAYERS. 



The apparent plasticity of the embryonic cells and layers and the comparative tardiness with 

 which they are clearly differentiated can not fail to impress anyone who follows closely the early 

 stages of development. The cell mass developed around the invaginate area, forming the thoracic- 

 abdominal process, can not be artificially divided into layers. It certainly represents very largely 

 the primitive mesoderm, but some of its elements pass to the opposite pole of the egg and become 

 almost indistinguishable from the superficial ectoderm, although it will be shown that they do not 

 pertain to this layer. A part of this mass remains as the mesoderm of the rudimentary abdomen, 

 while many of the cells which migrate from it degenerate and perform a nutritive fiinction. 



The endoderm does not appear as a definite layer until the egg-nauplius stage. It arises from 

 wandering cells which assume a peripheral position, and, joining the cells of the hindgut, form the 

 walls of the mesenteron. 



THE EYH. 



The optic discs appear as patches of ectoderm, one cell thick on either side of the long axis of 

 the embryo in front of the rudimentary abdomen. Before the appendages are definitely formed, 

 these have become thickened ectodermic discs. This thickening is due (1) to delaminatiou, or to 

 a division of cells in a plane parallel with the surface; (2) to emigration of cells from the surface, 

 due to crowding or to a division of superficial cells in a plane at ri.^ht angles to the surface. A disc 

 of cells is thus formed which gives rise chiefly to the eye and its ganglia. The cord of cells uniting 

 the two optic discs represents mainly the future brain. The eye proper is due to the differentiation 

 of the outer layer of the cells of this disc, while the ganglion is developed from the inner layer. 

 For fuller results of later studies not represented by these partial and preliminary notes, reference 

 must be made to Mr. Herrick's completed paper and to the summary of the whole history of the 

 development in the egg given at the end. 



