516 CELL DIVISION IN EGGS OF CREPIDULA. 



embryology, viz., the factors of embryonic differentiation. The earliest differ- 

 entiations of the fertilized egg are those which occur during cleavage; the differ- 

 entiations of cleavage lead to the formation of cells which are qualitatively and 

 quantitatively different, and if the causes of unequal and dissimilar cleavages 

 can be found, an important step will have been taken toward the solution of the 

 causes of embryonic differentiation. The facts presented above suggest that 

 the substances of the egg are stratified from the animal io the vegetal pole, 

 and that only ooplasm of a definite sort is given off in the ectomeres (micromeres) 

 leaving material of a different sort in the entomeres (macromeres) . 



On the other hand when the more fluid substances which normally lie at the 

 animal pole are thrown by centrifugal force into one of the first two blastomeres 

 leaving only yolk and a small amount of cytoplasm in the other blastomere, the 

 cell with the greater quantity of fluid substance gives rise to only three ectomeres, 

 while that with the smaller quantity also gives rise to three ectomeres (see 

 Conklin, 1912). Evidently therefore it is not merely the quantity of fluid 

 substance in the macromere, nor the stratification of the more dense and less 

 dense ooplasmic substances, which determines whether or not three ectomeres 

 shall be formed. 



It seems necessary to conclude as Lillie (1906) and Morgan (1910) have done 

 that the heavier or lighter substances of the egg, which may be stratified by 

 centrifugal force, are not morphogenetic substances, but are mere inclusions in 

 the real morphogenetic material. So far as yolk is concerned it is known that 

 it is a relatively dense inclusion which may be stratified at the distal pole of the 

 centrifuged egg, or may even be thrown out of the egg without seriously modifying 

 the morphogenetic processes. The same is true of the lighter oily or watery 

 substances which may be stratified or thrown out at the proximal pole of the 

 centrifuged egg. When both these denser and lighter inclusions are eliminated 

 there is left the clear substance of the middle zone in which the nucleus and 

 centrosome usually lie; and even the most of the material of this middle zone may 

 be transferred from one of the first two blastomeres to the other, without inter- 

 fering with the development of either blastomere. In what part of the cell, 

 then, does the polarity, symmetry, and morphogenetic organization reside? 



In Crepidula a nucleus, centrosome and sphere, plasma membrane and 

 probably a minimal quantity of the protoplasm of the middle zone (hyaloplasm) 

 must be present in a blastomere if it is to develop at all, and it must be in some 

 one or more of these parts, or in their relations to one another, that the organi- 

 zation resides. In my experiments the one visible thing which is left unchanged 

 in these centrifuged eggs is the axial relation between nucleus, centrosphere 

 and cortical layer. It is extremely difficult to move the mitotic figure by 

 centifugal force, — apparently it is anchored to the cortical layer by threads of 

 denser protoplasm, — and it is possible that a framework of such threads con- 

 nects nucleus, centrosphere and cortical layer, and thus preserves the axial 

 relation mentioned above. Such threads of denser protoplasm may possibly 



