202 



INTERNAL FACTORS 



IV. i 



Driesch, however, who has employed the same method as 

 Morgan and has corroborated his results, has been able to 

 produce, from the fusion of two ova, a larva single in all its 

 parts mesenchyme, skeleton, and gut. The organs are in 

 correct proportion but larger than the normal (Fig. 113). The 

 number of mesenchyme cells is about twice the normal. 



From this experiment it may of course be concluded that the 

 egg substance possesses a degree of isotropy, but not that that iso- 

 tropy is absolute. Nothing, it should be remarked, is known of 

 the way in which the eggs which fuse to form one embryo are 

 oriented upon one another. Garbowski, however, has stated 



FIG. 113. A. Normal gastrula of Sphaercchinus. B. Single gastrula 

 formed by the fusion of two blastulae. (After Driesch, from Korschelt 

 and Heider.) 



that fragments of different eggs of Echinus (in segmentation 

 stages) may be grafted on one another, and that the product 

 of their union will give rise to a normal embryo, whatever the 

 relative positions of the fragments. 



The fusion of blastulae to form giant Planulae has been 

 noticed by Metschnikoff in the medusa, Mitrocoma comae, and in 

 Ophryotrocka Korschelt has observed a similar fusion of distinct 

 eggs in the body cavity of the parent when the latter is herma- 

 phrodite. Lastly, Sala and Zur Stvassen have observed the 

 fusion in twos, threes, or more of the eggs of Ascaris, a result 

 experimentally produced by the first author by exposure to a low 

 temperature, while Zur Strassen has found that such double 

 eggs, provided they had united before fertilization and then been 

 fertilized by a single sperm, will produce perfectly normal embryos. 



