420 MExMOmS OF THE NATIONAL ACADEMY OF SCIENCES. 



appear at an earlier phase. The cleavage of the yolk is wholly, or almost wholly, confined to the 

 surface furrows, dividing planes rarely extending into the yolk below. My material was not ade- 

 quate for determining whether invagination was preceded by delamination or not, but seems to 

 render it highly probable that a delamination does not occur. 



My observations on Pontonia domeKtica, Palwmonetes vulgaris, and Hippa talpoides are very 

 fragmentary. In Pontonia 1 have one stage (Fig. 27) with three cells, one of which is dividing 

 with no sign of yolk division, and another with sixteen nuclei and corresponding yolk 

 pyramids. Here the conditions are precisely like those in Alpheus saulcyi at a similar stage, 

 and probably the segmentation of the two is similiar. In Palamonetes Faxon ( 17 ), relying 

 wholly upon surface views, states that segmentation of the egg begins in two planes almost 

 simultaneously. These planes arc at right angles to each other and pass through the long and 

 short axes of the egg. Whether this follows close upon the first division of the nucleus I have 

 not determined. I have an egg with two nuclei near the center of the undivided yolk, and a stage 

 with thirty-two yolk pyramids. This egg also agrees closely with that of Alpheus saulcyi at the 

 same stage. The nuclei are not quite at the extreme surface of the yolk. In the next phase when 

 sixty-four pyramids are present the protoplasm abuts on the surface (Fig. 24) of the egg. All the 

 protoplasm is distributed to the yolk pyramids and no delamination has taken place. 



With reference to Hippa I can only add the note that yolk pyramids normally occur, and in 

 the 64-cell stage (Figs. 1, 4) the lines of cleavage between adjacent segments are very distinct and 

 extend nearly to the center of the egg. At this stage all the protoplasm is ajjparently concen- 

 trated in these cells. Yolk pyramids similar in surface views to those of Hippa occur in CalUnectes 

 hastatus, Platyonychus ocellatus, and Lihinia canalicidata. 



Crangon vulgaris. — Kingsley (31) describes the segmentation of Crangon as follows: 



With the first segnietitation the protoplasm begins to leave its central position and seek the surface of the egg; 

 before the secouil division is completed it has reached the surface, leaving the yolk in the center. ♦ » * After the 

 second protoplasmic segnieutatiou is eflected the first segmentation furrows ajipear, the one following close upon the 

 other. The iirst to apjiear corresponds in its direction to the first nuclear division ; the second is at right angles to 

 it. * * * In Crangon, so far as I have been able to see, amoeboid cells reach the surface and take i)art in the 

 formation of the blastoderm before the process of gastrulation begins. In that form no yolk pyramids occur. 



Of cell division he says : 



In the process of cell division I h.ave never seen any traces of karyokinesis ; the division seems to be direct, and 

 affects first the nucleus and next the protoplasm. • » * In fact I do not recall a single statement of karyokinesis 

 being witnessed in decapod segmentation excepting in Astacus. 



On PI. I he gives a drawing (Fig. 3) of an egg with "about sixteen segmentation spheres." 

 Fig. 4 of the same plate represents a section of the egg shown in Fig. 3 and has six nuclei, five of 

 which are even with the surface, while one is near the center of the egg. The yolk sjiherules appear 

 to be fused, owing possibly to the disturbing eft'ect of the reagents employed. This central cell, 

 according to Kingsley, represents a portion of the egg protoplasm which is belated in its passage 

 to the suiface, but it divides and gives rise to cells which eventually reach the surface at a certain 

 point which marks the germinal area. Thus all the nuclei take part in the formation of the blasto- 

 derm, and the migration of the belated cells is completed before the "gastrula" invagination 

 occurs. I have made no observations on the very earliest phases of segmentation of Crangon, but I 

 have several eggs sectioned at the sixteen-cell stage, which ought therefore to correspond with 

 Fig. 4 of Kingsley's paper, but, on the contrary, they show a somewhat difl'ereut condition of 

 things. There are just sixteen nuclei present, all of which are peripheral or nearly so, and each 

 nucleus forms the center of a yolk pyramid, the cleavage planes of which are very marked and 

 extend more than half way to the center of the egg. Fig. 15, PI. xxvii, of the segmented egg ot 

 Alpheus, although belonging to a later phase, will fairly represent the condition of things which 

 we find in Crangon. None of the nuclei are tangent to the surface, but between them and the sur- 

 face there is still a considerable layer of yolk. Each is surrounded by a large mass of protoplasm, 

 which stains lightly with hajmotoxylon, and has the characteristic rayed appccarance. One of tiiese 

 nuclei is in the equatorial plate or metakinetic stage of division, and maybe represented very 

 nearly by Fig. 28, which shows a dividing cell in the egg of Alpheus at the same jieriod. As already 

 stated, the central yolk mass does not contain a single nucleus. The yolk is in the usual form of 



