MEMOIRS OF THE NATIONAL ACADEMY OF SOIENCES. 



343 



Seventh stage. — In three hours iuid three-quarters from the hist phase the bhistoderiiiic cells have 

 spread moie rapidly at a piveu point ou the egg, which loses its radial symmetry in consequence. 

 There is thus formed the embryonic area or first trace of the embryo proper. 



Eighth or inragi nation stage. — Three and a half hours later a portion of the blastoderm in the em- 

 bryonic area is invaginated, that is to say, some of its cells pass below the surface in a bodj', and 

 the invagination stage is reached. The invagination is solid, or nearly so, as is the case with 

 nearly all Decapods. Fig. 9 represents an oblique section of one end of an egg, through the area 

 of invagination, Ig. The epiblastic cells contain small oval nuclei'. There are no yolk cells in the 

 interior of the egg. The vitellus is here segmented into large, irregnlas fragments, each of which 

 is composed of yolk corpuscles similar to those seen in Fig. 4. It is just i>ossible that this fracture 

 of the yolk, which is commonly seen in the eggs of other Crustacea, is artificially produce<l at 

 lea.st to some extent, at this stage. 



Ninth stage. — After another period of three and a half hours, while the external change is not 

 marked, the invaginated <^ells have rapidly nuiltiplied and given rise to a considerable cell-mass 

 below the surface at that point. 



Tenth stage. — In thirteen and a half hours from the last stage, or when the embryo is fifty-two 

 hours old, important changes have been etiected. In surface view the embryo presents a heart- 

 shaped or somewhat three-sided area. The optic discs appear as widely separated patches of 

 ectoderm, united to the thoracic-abdomuial plate, a mass of cells which forms around, but chiefly 

 in front of, the point of ingrowth. Sections through this egg show a considerable thickening in the 

 optic discs, and an accumulation of large granular cells in the abdominal area. These latter un- 

 doubtedly represent some of the primitive mesoderm and endoderm. 



The phenomena just recorded are given iu a more condensed form in the following table, 

 which shows the age and corresponding growth of the embryos at the successive stages. The age 

 of the first stage is assumed to be 6 hours, which is probably not far from the truth. 



In the above account we are constantly dealing with dift'erent eggs, and assume of course that 

 they are all at any given time iu the same phase of development. While this is not strictly true, 

 it is very nearly so. The eggs are at first about on a par, and it is only later that some become 

 handicapped, producing those slight diflerences which may be seen in embryos from the same 

 female.* 



Time of hatcbiDg Jane 4, a.m., early. Tetnperatore SO'^ F. Diameterof egg ^ inch. 



We thus have in Stenopus a typo of the so-called "centrolecythal" segmentation, exactly 

 comparable to that of Penams, and essentially like that which is probably characteristic of a 

 large number of the Decapod Crustacea. The fact that all the inotoplasm of the egg enters into 

 the blastoderm and that no yolk cells are now formed, is of some interest, and this subject, will 

 be considered more fully in a paper on the develo[)ment of Alpheus. 



* This is not trne of the American lobster, Homarun americanus, in which I have made a very complete study of 

 tlie segmentation process. In a batch of segmenting lobster's eggs, there is a decided Lick of uniformity. Some ova 

 which afterwards continue to develop, remain with yolk unsegmented until the third or fourth day after fertilization. 



