20 COMMISSION OF CONSERVATION 



cells, to some extent resembling two cups set one within the other, but 

 continuous at their rims, the embryo begins to assume a low grade of tissue 

 organization, for the outer layer naturally has to contend with the more 

 physical relations of the external world, while the inner takes up the more 

 chemical work of digestion. A portion of the yolk was distributed to the 

 protoplasm of each cell as it was formed, but the richest supply was 

 divided up among the endoderm cells. Continued cell-division now 

 deepens this endoderm into a primitive gastric cavity (archenteron, 

 coelenteron, Plate V, fig. 21), as well as extends the ectoderm around to 

 narrow the blastopore. The polar bodies may still remain and serve as a 

 landmark for orientation; but, due to the most rapid division in the 

 region of the smaller of the first pair of blastomeres, the polar bodies have 

 come to be carried in the opposite direction, and the long axis of the em- 

 bryo begins to change from vertical to transverse. Cilia appear on the 

 ectoderm cells (Plate I, fig. 9; Plate V, fig. 21) and the little organism 

 now undertakes swimming movements characteristic of a post-embryonic 

 period of life, distinguishing it as a larva. 



Division of cells must be considered to be alwaj's preceded and governed by nuclear 

 division analogous to that referred to in the formation of the polar bodies; in fact the 

 presence of a nucleus is the surest proof of cellular structure. The nuclear spindle is formed 

 in the direction of the longest axis of the protoplasmic mass and the jlane of cleavage 

 of the protoplasm is transverse, through the centre of the nuclear spindle. Judged by 

 this test it would seem uncertain whether the body originating in the first cleavage and 

 hitherto spoken of as the deutomere is a true blastomere. Brooks, whose description of 

 the external features of segmentation seems to be most complete, was unable to follow 

 the phenomena of nuclear division. In his work of 1880 he speaks of this mass as a 

 macromere in contradistinction to the two small cells (micromeres). Nelson (1901), 

 who devoted much time to nuclear structures, speaks of the same mass as a yolk-sphere. 

 Neither figure for it nor assign to it a nucleus of its own, although Nelson in one place 

 says, "There seems to be some evidence of a yolk nucleus, which is presumably the 

 relic of one of the pronuclear astrospheres. It is hoped that the doubtful points will 

 be cleared up by future studies." Its long connection with one of the blastomeres 

 would seem to indicate that it does not itself possess initiative, and the behaviour of 

 this blastomere would suggest that it finds the yolk a burden. 



Another point for consideration is the direction of the first cleavage planes. Kors- 

 chelt and Heider (1900) give diagrams (Part IV, fig. 11. My Plate V, fig. 16) il- 

 lustrating cleavage in Lamellibranchs as, first, horizontal, forming a micromere above 

 and a macromere below; second, vertical, dividing the micromere; third, a micromere 

 is obliquely separated above from the macromere, which from the first has had a nucleus 

 of its own. The first cleavage as figured by Horst (1884, Plate I, fig. 3), if he purposely 

 oriented it transversely (Plate V, fig. 17), agrees with this, but Nelson (1901, fig. 80) 

 represents the first cleavage as vertical (similarly to my fig. 11). 



A question of some interest, and possibly correlated with both of the preceding, 

 is how far gravitation can affect segmentation. There is a polarity in the distribution 

 of the protoplasm and the yolk, the more pure protoplasm being at the animal pole and 

 the heavier yolk-laden protoplasm at the vegetative pole; the first activity and the polar 

 bodies appear above; the oosperm lengthens vertically before dividing; the nuclear 

 spindles and planes of cleavage, at least for the first few times, have a simple relation 

 to the direction of the chief axis, in that they are either parallel or transverse to it. It 

 seems scarcely possible that such a constant and considerable force as gravitation could 

 fail to have an influence. On the other hand the internal forces of the oosperm are 

 stronger, at least during the time of their activity. Oosperms floating in the water or 

 resting on a solid surface may be spherical instead of deep or flattened ; unfertilized eggs 

 fresh from the ovary are usually elongated in the principal axis, although that axis is 

 not vertical for all or even a large proportion of them while in the ovary. In this latter 

 case the eggs are more immediately influenced by their relation to the wall of the fol- 



