422 MEMOIRS OP THE NATIONAL ACADEMY OF SCIENCES. 



involved. This remark also applies to Stenopns, where eight blastomeres or yolk pyramids appear 

 at the beginniug of the third phase. In Homarus there is less uniformity in the appearance of the 

 superficial segments, since the cells do not migrate to all parts of the surface at a uniform rate. 



Lereboullet (36) described the early stages of the segmenting egg of the crayfish Astactis 

 fluviatilis in 1862. His account, though somewhat vague and unsatisfactory owing to the tech- 

 lucal difficulties under which he labored, is confirmed in essential particulars by a later observer, 

 Skimkewitch (56), who has given a short description, without figures, of the process in the Eussian 

 crayfish AstacMS leptodaciylus. His account is briefly as follows: The cleavage nucleus with its 

 protoplasm passes from the center to the surface of the egg and undergoes segmentation. The 

 resulting nuclei, the exact number of which is not given, then diffuse over the surface of the egg 

 and the yolk segments around them. The cleavage planes between adjacent pyramids, at first 

 extend only half way to the center of the egg and never quite reach it. There remains at the close 

 of the segmentation a small mass of undivided yolk (" dotierlcern^') lying in the center of the ovum. 



According to these observations then, Astacus offers us an example of centrolecithal segmen- 

 tation in the most exact sense of the term. It seems to me quite probable that this migration of 

 the protoplasm may be only apparent, and that the segmentation may proceed much as it does in 

 the lobster. 



Brooks, in his memoir on Lucifer (8), gives an account of a segmentation which differs in 

 some particulars from that of any macrouran which has been studied. The Lucifer egg undergoes a 

 total and perfectly regular segmentation, and in the first stages may be compared with Eupagurus. 

 In Lucifer, however, a segmentation cavity is formed, which can be clearly seen when sixteen 

 spherules are present. At this time one pole of the egg becomes flattened, and one of the spherules 

 in the area, which has become differentiated by the possession of food yolk, is gradually invagi- 

 nated into the segmentation cavity. Meantime the invaginated cell divides; other changes ensue 

 which lead to the infolding of more cells and the formation of a two-layered embryo, the "gastrula." . 

 Brooks thinks that th<3 yolii bearing cell represents a yolk pyramid, and that after a longitudinal 

 division each half divides transversely or delaminates, and that the other two inner cells contain the 

 yolk, while the outer products remain at the surface and form a part of the "blastoderm" (or eudo- 

 derniic invagination). This last point, however, was not settled. In Brooks's view the segmenta- 

 tion in Lucifer is a secondary modification of the yolk pyramid type, and this has been brought about 

 "by the gradual reduction of the amount of food material and its restriction, at last, to a single 

 one of the cells of the segmenting egg." He says that, while Lucifer is undoubtedly a very primi- 

 tive Malacostracan, it can hardly be regarded as a primitive Crustacean; and since we meet with 

 abundant examples of centrolecithal segmentation both above and below Lucifer, we can not regard 

 the Lucifer egg as ancestral or as the unmodified descendant of an ancestral type of egg. "We 

 must, therefore, believe that the egg of Lucifer has been simplified by the loss of the greater part 

 of its food yolk." 



It is to be regretted that Professor Brooks did not find material sufficiently abundant to war- 

 rant the sacrifice of some of the segmenting eggs for sections; for until this is done we can not be 

 certain of the origin and relations of the mesoblast and entoblast in this extremely interesting 

 species. A careful study of the subject both in Lucifer and Sergestes would form a very valuable 

 and much needed contribution.* 



It seems to me quite probable that we have in Lucifer a repetition of what occurs in Homarus 

 and Alpheus, and that the yolk-bearing cell corresponds to the inwauderitig or delaminated cells, 

 which occur in the lobster at the close of segmentation. In each case they arise by transverse 

 dinsion from the superficial cells of the blastophere. In each case, also, they appear just before 

 invagination, and migrate into the segmentation cavity. The differences are that in the lobster, 

 for instance, the segmentation cavity is already filled with yolk ; the centripetal cells are numerous, 

 and they do not necessarily come from that point on the surface which corresponds to the point 

 of invagination or ingrowth. As to the comparative history of the two cells in the two cases little 



* Since tbis was writteii Professor Brooks has anuouiicecl in his report of the work of the marine laboratory of 

 the Johns Hopkins University, that he has recently obtained the eggaot Lucifer in abundance at Jamaica, and is now 

 engaged in studying its embryology. 



