ONTOGENESIS 189 



In certain species a similar sequence of events 

 occurs as a natural incident of development. Some 

 forms of parasitic wasps lay eggs within the bodies 

 of caterpillars, the young larvae feeding on the cater- 

 pillar tissues, and finally emerging to spin cocoons on 

 the surface, within which they carry out their final 

 metamorphosis. It has been discovered that only 

 one egg may be laid thus within the caterpillar, but 

 that it fragments into scores or hundreds of por- 

 tions, from each of which a perfect parasitic wasp 

 develops. This is known as polyembryony. To 

 recur again to the sea-urchin blastula, mentioned 

 above, it is clear that the factor which determines 

 whether one individual or two is to be the result of 

 the process of development lies somehow in the re- 

 ciprocal relation of the parts of the blastula. This 

 may be illustrated in another way. If the zygote, 

 after the first cleavage, be placed in sea- water which 

 lacks calcium, the two cells will separate, and, if 

 replaced in normal sea-water, each will develop into 

 a perfect individual. As in the previous case, two 

 individuals arise from one zygote. Here it is clear 

 that the absence of contact with the one blastomere 

 determines that the substance of the other shall 

 differentiate into one individual instead of into half 

 a one, as it ordinarily would do. The result is, how- 

 ever, always the specific type. 



Heteromorphosis. Although the regeneration of 

 a new appendage or other part in nearly every case 

 conforms to the specific type, and the process is 



