MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



343 



Seventh stage. In three hours and three-quarters from the last phase the blastoderruk; cells have 

 spread uiore rapidly at a given point on 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 imagination stage. Three and a half hours later a portion of the blastoderm iuthe em- 

 bryonic area is iuvaginated, that is to say, some of its cells pass below the surface in a body, and 

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

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

 of invagiuation, Ig. The epiblastie cells contain small oval nuclei. There are no yolk cells in the 

 interior of the egg. The vitellus is liere segmented into large, irregular fragments, each of which 

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

 of the yolk, which is commonly seen in the eggs of other Crustacea, is artificially produced at 

 least 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 cells have rapidly multiplied 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 effected. 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-abdominal 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 au 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 in 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 different eggs, and assume of course that 

 they are all at any given time in 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 differences which may be seen in embryos from the same 

 female.* 



Time of batching June 4, a. m., early. Temperature 80 F. Diameter of egg fa inch. 



We thus have in Stenopus a type of the so-called "centro-lecythal" segmentation, exactly 

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

 large number of the Decapod Crustacea. The fact that all the protoplasm 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 development of Alpheus. 



" This is not true of the American lobster, Homarus americanits, in which I have made a very complete study of 

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

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



