MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 421 



large spherules on angular blocks, which are abundantly perforated with round lacuna?. Crangon 

 agrees essentially at this stage with at least three other species, namely, with Alpheus saulcyi, 

 Pontonia domestlca, and Stenopw-s hispidus. 



It is probable that Figs. 3 and 4 of Kingsley's paper do not correspond, the latter representing 

 the older egg, that is, older than the sixteen-cell stage. In the segmenting eggs of Homarus there 

 is a striking individual variation, which finds expression chiefly in the external characters of seg- 

 mentation, but I have never observed this in any related forms, and can not say to what extent it 

 occurs. Judging from analogy, I think it will be found that as a rule all the protoplasm in the egg 

 of Crangon vulgaris reaches, or very nearly reaches, the surface, as in Homarus and A Ijiln'iin xaulcyi, 

 and that toward the close of the yolk-pyramid stage delamination occurs and some of it wanders 

 back into the depths of the yolk. If this is true, the "belated" cell near the center of Fig. 4 of 

 Kingsley's paper may represent some of the protoplasm which has taken this roundabout journey. 



[While this memoir was in press a paper has appeared by W. F. It. Weldon, on "The Forma- 

 tion of the Germ-layers in Crangou vulgaris." (Quart. Jour. Mic. Sci., Vol. xxxni, pt. 3, March, 

 1892). He makes no mention of the budding of cells from the blastoderm before iuvagination, 

 nor of the presence of migrating cells at a later period not derived from the invaginate cells or 

 ventral plate, hence it is probable that in Crangon primary yolk cells do not occur. In this case 

 the suggestion just offered does not let us out of the difficulty.] I have not succeeded in obtaining 

 the segmenting eggs of Calliauassa for comparison with the early stages of Callianassa meiliterranea, 

 described by Mereschkowski (40). It seems to me, however, that the diagrams in Mereschkow- 

 ski's paper are misleading, and that the process of segmentation in Callianassa, instead of being 

 peculiar as one might infer, is essentially typical. According to this observer's account the "blas- 

 toderm" arises without yolk segmentation. Nuclear division is at first central, and the resulting 

 cells, sixteen in number, pass gradually to the surface and form a deep layer of protoplasm 

 inclosing the yolk. This layer, at first raised into hillocks corresponding to the nuclei, finally seg- 

 ments into sixteen parts around the latter. These segments are formed simultaneously over the 

 egg, but the yolk is not involved. The cells multiply rapidly and form a layer of tall prismatic 

 elements, which gradually flatten out as division proceeds, into ordinary blastoderm cells. 



If there is any analogy between this egg and that of related forms the broad "protoplasmic" 

 layer is really the protoplasm plus the peripheral yolk. It would be remarkable in any case if 

 a segmenting egg could acquire such a mass of protoplasm, not to speak of the suddenness with 

 which the acquisition is made. That this layer, comprising more than oue-haif the contents of the 

 egg, is largely yolk is indicated by the fact that the nuclei which occur in it are represented as 

 surrounded by a protoplasmic reticulum as normally occurs. If this is true, the prismatic cells are 

 yolk pyramids, and their line of separation from the central yolk is purely imaginary. 



Ishikawa found, in his studies upon a Japanese prawn, Atyephyra compressa (27), that the egg 

 underwent at first a total and regular segmentation. At the end of the fourth phase the yolk is 

 centralized and the protoplasm surrounds it, and in the next stage, after thirty-two blastomeres 

 have been formed, the central end of each separates oft' to form a yolk segment. The yolk seg- 

 ments, which fill the center of the egg and correspond to the common yolk mass with which the 

 apices of the yolk pyramids blend in other Decapods, are of unequal size and contain nuclei which 

 do not take part in the "blastoderm." These nuclei probably correspond to the delaminated cells 

 of Homarus and Alpheus. In the latter they appear at the close of segmentation. It is quite 

 probable that the time at which these cells are budded off may vary considerably iu different 

 species. 



In Eupagurus prideauxii (39) Mayer found that the protoplasm segmented first iu the center 

 of the egg, as in other forms, until eight nuclei were present. When this stage is reached the yolk 

 now segments not simultaneously into eight blastomeres, as iu the case with the Isopod Asellus 

 aquaticus, but according to its inherited primitive manner, first into two, then into four and eight 

 spherules. Segmentation of the yolk is thus at first total, but after the fourth phase the spheres 

 unite in a central yolk core as in other forms. 



In Alphas saulcyi I did not find any eggs which showed a progressive segmentation of the 

 yolk between the stages represented in Figs. 9 and 10, and hence I infer, as already stated, that 

 the segmentation of the yolk is there a simultaneous process for each of the sixteen segments 



