238 OSCAR RIDDLE 



of most extraordinary size, the writer can not but wonder if there 

 existed a causal nexus between the extraordinary^ size, on one 

 hand, and the unusual separation of the blastomeres on the other. 



Plainly the main question is. Why, or by what means, is 'inde- 

 pendent' development instead of coordinated, mutual, integrated 

 development initiated in the two blastomeres? One means 

 already known for obtaining this independent development' is 

 that of physical separation of the blastomeres. Surely, the 

 exact placement and position and inclination of the early blasto- 

 meres (meroblastic eggs) are not wholly out of reference to the 

 size and to the consequent polar configuration. And surely the 

 type of cleavage, normal to normal blastula-formation, etc., is 

 not out of reference to the normal size and shape of the ovum. 

 A somewhat unusual disposition of the segmentation spheres at 

 the animal pole — these being, at their outer borders, abnormally 

 raised in extraordinarily large eggs and abnormally lowered in 

 extraordinarily small ones — would thus seem to afford a possible 

 clue to this relatively rare occurrence. 



In holoblastic eggs the egg-size might still be the conditioning 

 factor, as in the case just noted of meroblastic eggs; for, although 

 the blastomeres there are always in apposition, the centers of 

 metabolic activity (nuclei, centrosomes, etc.) in abnormally large 

 eggs would be separated to a degree unusual to the species, and 

 thus conceivably afford a basis for the ' independent action' of the 

 first two segmentation spheres. Abnormally small ova, in divi- 

 sion, would provide two cells with abnormally (for the species) 

 large surface areas in proportion to their masses, and con- 

 ceivably this may similarly result in the immediate assumption of 

 'independent development' in each blastomere. 



According to the view just sketched, identical twins should arise 

 from the extremely large and the extremely small eggs of a species. 

 Presumably such would be produced in approximately equal 

 numbers. According to the theory of sex hitherto developed by 

 the writer, males should develop from the smallest and females 

 from the largest eggs. Apparently, the same size relations should 

 hold for the sexes in twin-producing eggs. The two cases of 

 identical twins described in this paper are two instances in sup- 



