342 



SCIENCE. 



[N. S. Vol. XXIII. No. 583. 



in all cases except where the point of entrance 

 happens to lie in this meridian. No evidence 

 for such movement is given; indeed, it is by 

 no means demonstrated that the spermatozoon 

 alvcays moves in a meridian of the egg. The 

 movements of the spermatozoon constitute the 

 only indications that bilateral organization 

 exists before fertilization and they seem to the 

 writer to oppose rather than to support the 

 conclusions drawn from them. 



As the sperm nucleus moves to the posterior 

 pole the clear and the yellow protoplasm move 

 with it; the latter collects into a yellow crescent 

 with its middle at the posterior pole and its horns 

 extending about half way around the egg just 

 below the equator. This position it retains 

 throughout the whole development, giving rise to 

 the muscle and mesenchyme cells. 



After the sperm and egg nuclei have met at 

 the posterior pole they move in toward the center 

 of the egg and the clear protoplasm goes with 

 them; the only place where the latter remains in 

 contact with the surface is along the upper border 

 of the crescent. At the close of the first cleavage 

 the nuclei and clear protoplasm move into the 

 upper hemisphere, and thereafter, throughout de- 

 velopment, this hemisphere contains most of the 

 clear protoplasm and gives rise to the ectoderm. 



The yolk which before maturation was central 

 in position is shifted toward the animal pole when 

 the protoplasm flows down to meet the sperma- 

 tozoon; when the sperm nucleus and surrounding 

 protoplasm move to the posterior pole the yolk 

 is moved down around the anterior side of the egg 

 to the lower pole, and when the clear protoplasm 

 moves into the upper hemisphere of the yolk is 

 largely collected in the lower hemisphere. This 

 yolk-rich area gives rise to the endoderm. 



At the end of the first cleavage the chorda 

 and neural plate areas are visibly different 

 from surrounding regions, since they contain 

 less yolk. Later muscle and mesenchyme be- 

 come distinguishable, the former being deep 

 yellow, the latter light yellow or clear. 



These are the most important facts regard- 

 ing the ' organ-forming substances.' 



Before turning to Conklin's general con- 

 clusions a brief consideration of the grounds 

 for believing that the differentiated regions 

 represent formative substances is necessary. 

 And first, what are the formative substances? 

 The visible differentiations of the egg are due 



not to visible differences in the protoplasm 

 itself, but to the localization of the inert sub- 

 stances, yolk and the yellow granules. Conk- 

 lin does not regard these inert substances as 

 formative, but apparently believes their local- 

 ization indicates a corresponding localization 

 of different ' kinds of protoplasm.' We are 

 justified in inferring from the presence of 

 different inert substances that different kinds 

 of activity have occurred in the past, but cer- 

 tainly a single ' kind ' of so complex a sub- 

 stance as protoplasm is- capable of various ac- 

 tivities under different conditions. Moreover, 

 the significance of inert substances in proto- 

 plasm is pi'imarily retrospective, not pro- 

 spective. 



The protoplasm containing yellow granules 

 in the Cynthia egg gives rise to muscle and 

 mesenchyme, according to Conklin. Yet the 

 yellow granules are not confined to this region, 

 but appear about all the nuclei during cleav- 

 age, about the nuclei of the test cells, and in 

 the viscera of the adult. In other words, there 

 is no indication that this region contains any 

 specific kind of protoplasm not found else- 

 where. During ovarian stages the test cells 

 invade the peripheral layer of the egg. It 

 seems at least not improbable that the yellow 

 granules are associated with the earlier pres- 

 ence of the test cells. 



The yolk spherules are similarly inert ; their 

 localization in the egg can be as readily ex- 

 plained on physical grounds as by postulating 

 a specific kind of protoplasm in the region 

 where they exist. It is quite probable that 

 the presence of yolk granules determines spe- 

 cial activities in the protoplasm about them, 

 and indeed it is not unlikely that the yolk 

 itself is the important entodermal formative 

 substance. But that there is no special 

 formative substance corresponding to the yolk 

 region is indicated by the fact that parts of it 

 go to other than entodermal regions. 



The fact that the clear protoplasm from the 

 nucleus and the yellow protoplasm move down- 

 ward to meet the sperm and accompany it in 

 its movements does not necessarily indicate 

 anything more than greater mobility of these 

 areas in consequence of the absence of yolk. 

 In certain other eggs, where no such areas 



