342 MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 



another egg of the same phase neither cell is quite at the surface, so that the example ^iven in 

 Fig. 1 may be taken to illustrate a tendency, not a rule.* The yolk (Fig. 1, Y. C.) consists here, 

 as in subsequent stages, of homogeneous and tolorably unifoim green corpuscles. No vacuolar 

 cavities are to be seen. 



Second stage. — Four and one-half hours later the second segmentation is going on or is already 

 completed. The egg from which Fig. 2 was taken contains four nuclei with perinuclear protoplasm. 

 It was cut into a series of .50 sections, of which the second represented is the twenty-first. Nuclei 

 occur in .sections 21, 25, 20, and 35, none being as yet superficial. A portion of section 21 (Fig. 2) 

 is shown under a higher power in Pig. 3. 



Third xtaf/e. — After three hours and twenty-five minutes have passed the third phase is reached 

 and we have eight cells, around which the yolk is superficially constricted into eight corresponding 

 lobes or segments. A surface view of thi.s entire egg and also a section through it are shown in PI. 

 VI. Figs. 5 and 6, and a tangential section of one of the nuclei and lobes is given with more detail in 

 Fig. 4. The constriction furrows appear to be considerably deeper than they actually are, and we 

 might predicate a total segmentation of this egg without the knowledge which the section affords. 

 We have here a merely superficial indentation of the yolk, the great central mass of which is undi- 

 vided. ' It is a close approach to the yolk pyramid stage seen in Astacus, Alpheus, Hippa, PaliB- 

 monetes, and many other Decapods. The dividing^ planes, Figs. 7 and 8 (unless artificially pro- 

 duced), do not penetrate into the egg. The furrows extend inward to a plane just below or on a 

 level with the nucleus. 



Each nucleus with its outer protoplasm may be spoken of as the cell, and it is hardly probable 

 that there is any protoplasm like that surrounding the nucleus in the other parts of the egg. The 

 nuclei increase gradually in .size, as seen by comparing the figures of successive stages, and the 

 surrounding plasm, which they manufacture out of the yolk, is also of greater bulk. Each is a 

 fiattened, oval disc, shown well in transverse section in Fig. 5 at a, and tangeutially in Fig. 4. 

 It contains coarse grains and granules of chromatin, and the enveloping protoplasm radiates visibly 

 but a short distance between the yolk siiherules. The long axis of each nucleus lies in a plane 

 parallel with the surface. Cell multiplication is in all cases indirect, as my observations show to 

 be the case with several other related forms, and this is undoubtedly the rule not only with the seg- 

 menting eggs of the Decapod Crustacea, but with those of all the Metazoa. There seems to be an 

 exception in the case of A Ipheux minor. 



Fourth sttu/e. — After another interval of an hour and five minutes there are sixteen cells re- 

 sulting from the fourth segmentation. The blastoiiiercs are less sharply marked at the surface and 

 more distinctly polygonal. Six nuclei are cut by the section given in Fig. 7. They are nearer to 

 the surface than in the former stage. 



Fifth stage. — The egg represented by Fig. 8 is three hours older than the last and has thirty- 

 two cells and the same number of superficial segments. Up to this time the egg has exhibited 

 radial symmetry. The nuclei are quite near the surface of the egg. They are more spherical and 

 the investing protoplasm is less conspicuous than formerly. The fissures between contiguous 

 blastomeres are becoming le.ss and less prominent. 



Sixth stage. — After a longer period, nine hours and forty-five minutes, the process of regular 

 division into smaller and smaller superficial .segments has proceeded until 128-256 of these bodies 

 are formed. The cells lie at the surface, just under the chorion, and form a continuous envelope, the 

 primitive blastoderm about the central yolk. This yolk mass is not segmented, nor does it include 

 any nuclei which have not participated in forming the blastoderm. In one or two instances a cell 

 was observed just below the service. This may be interpreted as either having never reached the 

 surface or as having been there and moved below it towards the interior. But the general state- 

 ment is doubtless true that all cells reach the surface, and that there is no extensive migration to 

 the interior, as there is in Alpheus, before invagination. 



• It now seems probable to me tb.at tliis superficial cell represents the male .and tbe central cell the female pro- 

 nuclens. A small, deeply st.iining body, which I interpret as au undoubted polar cell (not shown in Fig. 1), lies 

 underneath the chorion, not far from the superficial cell. 



