22 COMMISSION OF CONSERVATION 



the proportion of active protoplasm, and, as gravitation adds weight and pressure, 

 tends to sag into the lower portions and give an equipoise to the floating oosperm. 

 "The pure, soft, active protoplasm above flattens across the top and forms a transverse 

 spindle, resulting in an attempt at vertical fission, the only way of effecting an equal 

 division without separating the more active protoplasm from the yolk. The horizontal 

 constriction results from the downward gravitation of the heavier, stiffer yolk below, 

 combined with the upward buoyancy, flattening, division, and rounding up of blasto- 

 meres above. The first segmentation nucleus divides into two daughter nuclei that go 

 into the two blastomeres, leaving the deutomere for an interval without nucleus. This 

 does not become independent, but remains under control of one blastomere and its nu- 

 cleus, as shown in the complete re-union during the resting stage when the segmenting 

 forces are relaxed, and in the fluidizing and absorption of some of the yolk. The second 

 fission follows the same impulses as the first, each daughter nucleus dividing into two 

 grand-daughter nuclei, one for each of the four blastomeres, the yolk still remaining 

 dependent. The later formed blastomeres may be more hampered by their greater 

 proportion of yolk, the earlier formed ones precede them in division, which contributes 

 along with the deutomere to the apparent bilateral symmetry. Similar phenomena in 

 segmentation are shown by Cardium pygmceum, Anodon piscinalis, and Nassa muta- 

 bilis. 



Having arrived at the termination of embryonic life, we are in a position to better 

 understand the fate of the yolk. In the restricted sense the term yolk includes the 

 whole collection of granules of concentrated food-matter suspended in the egg proto- 

 plasm. It is itself passive, and in fact somewhat in the way of any activity on the part 

 of the protoplasm lying around and between the granules. Both together are spoken 

 of as deutoplasm, and occupy the nutritive hemisphere, which in the first cleavage be- 

 comes largely separated from the blastomeres as a deutomere. The blastomeres of the 

 earlier stages (or their products by cleavage) become the ectoderm of later stages. The 

 deutomere (or the remnant of it) becomes endoderm. There arises the question as to 

 whether the original blastomeres, by division, give rise to the whole of the ectoderm, 

 thinning out and stretching around the undivided deutomere. This can not be, for the 

 deutoplasm is at first a large mass and later becomes relatively small. Then comes the 

 question of how the reduction takes place. It might occur through absorption into some 

 of the actively dividing blastomeres, which could only be to a limited extent, since it 

 would have to pass by way of that one which remains in contact. Besides, towards 

 the end of segmentation the reduced deutomere itself divides to form the endoderm, 

 but where does its nucleus come from? If the diagram by Korschelt and Heider were 

 to hold good for the oyster the explanation would be forth-coming that one of the first 

 daughter nuclei has remained quiescent all this time, while the other has been active. 

 It is hardly likely that such a thing would happen, or that the nucleus would remain 

 unobserved. Moreover, that the diagram does hold good is improbable. There re- 

 mains the possibility that the blastomere which retains connection with the deutomere, 

 keeps parcelling out deutoplasm to each new blastomere, formed by division of itself, 

 until it finally merges with the reduced deutomere and effects an equal division. In 

 other words, from the very first fission, the deutomere and one blastomere together form 

 a macromere with one nucleus. The blastomere partially separates from the deutomere, 

 divides off a free blastomere and again merges with the deutomere to form a macromere. 

 All of the free blastomeres are micromeres and continue to segment to form the ecto- 

 derm. The deutomere becomes reduced until, at the final merging with the attached 

 blastomere, the nucleus of the latter gains complete control of the reduced deutoplasm, 

 and is able to effect an equal division to begin the mesoderm. This finds a nucleus for 

 every cell and explains all the phenomena, the polarity, the influence of the yolk, the 

 irregularity of the cleavage and in the embryonic stages, the peculiar behaviour of one 

 blastomere and of the deutomere, the fluidization of the central protoplasm of the latter, 

 the position of the nuclei, the alternation of active and resting periods, the increase in 

 volume and number of the micromeres, the reduction in mass of the macromere, the dis- 

 tribution of yolk and its gradual conversion into protoplasm, the late origin of the endo- 

 derm, the formation of the gastrula by epibole. 



