MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 335 



from the first nucleus of the fertilized egg, through all the einbryouic and larval stages, up to the 

 adult condition. The eggs of each of the thirteen species whicih occur in the Bahamas were ob- 

 tained and studied sufficiently to ascertain what are the si)ecific dittorences in development, and 

 four species were studied exhaustively, in detiil. These four are Alpheus heterochelin, Say ; A. wit- 

 «M«, Say; A. saulcyi, nwd the Bahama Ac<eroc7tt'/(.s-. Uidess otherwise stated the following note.s 

 on the early stages refer to the hist species. The development in the egg is the same for all, except- 

 ing A. minor, which will be referred to separately. 



This prawn has proved to bo a good subject in which to study the origin and role of certain 

 much disputed bodies, which are met with in several Crustacea, the "secondary mesoderm cells." 



SEGMENTATION OK TUE NUCLEUS AND OF THE YOLK. 



The egg when laid, is enveloped by a single membrane, the chorion or shell, to which is added 

 the secondary membrane of attachment. If ihe nucleus is unfertilized, it is not able to initiate the 

 process of segmentation. The fertile nucleus divides, and its products pass towards the surface, 

 until a syncytium of eight nuclei is formed. Either just before or after the division of these, 

 the yolk undergoes segmentation simultaneously over the whole surface into a similar number of 

 partial pyramids. Each yolk pyramid has a large nucleus at its base, while its apex fuses with 

 the common yolk mass in the interior of the egg. The process is now a regular one until 128 to 25(j 

 small segments are formed. The rate of cell multiplication is then retarded over one half of the 

 egg, while it still continues and perhaps is accelerated over the remaining portion of it. The egg 

 thus loses its radial symmetry and becomes two-sided. It is important to notice that no products 

 of the segmentation nucleus are left in the interior of the yolk. The superficial i)yramidal 

 structure is lost; the primitive blastoderm is established, and there now takes place a general 

 migration of nuclei from flie surface to the yolk within, but principally, as would be exjjccted, 

 from that part of the egg where the blastoderm cells are most numerous, corresponding to the 

 future embryo. This is followed by a partial secondary segmentation of the food yolk info balls. 

 The yolk-ball is apparently fprmed about the migrating nucleus, bat as the latter is moving, this 

 segmentation is irregular. 



Mr. Herrick has been able to follow very closely the entire process of segmentation in Stenopus, 

 where it is substantially the same as that just described, except that there is no general migration of 

 cells from the surface, prior to invagination. This is also true of Poutonia domestica, and it is quite 

 probable that the majority of niacroura pass through the same phases in their early development. 



Alpheus minor is anomalous from the fact that the products of the first nucleus instead of 

 multiplying by regular binary division, multiply indirectly, and give rise to numerous nuclei, 

 many of which degenerate, before the blastoderm is formed. 



THE INVAGINATION STAGE. 



A slight invagination occurs where the superficial cells are thickest, and the egg becomes 

 what has been generally regarded as a modified gastrula. The depression is shallow, and does 

 not form an inclosed chamber within the yolk. The included cells multiply rapidly, and form a 

 mass of nearly similar elements, some of which pass into the yolk. The protoplasm surrounding the 

 nuclei of these cells is |)rolonged into a reticulum, which encloses myriads of small yolk frag- 

 ments, and probably digests them by an intracellular process, after the manner of feeding amiebie. 

 The thickening in front of and surrounding the pit, which is now obscured, is the rudiment of the 

 abdomen. Anteriorly the " procephalic lobes" or more properly the optic disks make theirappear- 

 ance on either side of the long axis of the embryo, as circular patches of ectoderm. Meantime 

 nuclei wander from the cell mass below tlie abdominal plate to all parts of the egg. Some pass to 

 the opposite side, and take np a position beside the flattened epithelial cells, of what was the i»rlm- 

 itive blastoderm. The majority, however, pass forward and u[)ward in divergent lines from the 

 sides of the abdominal plate, and eventually large numbers of these wandering cells settle down 

 over the dorsal surface of the embrj'o. 



