80 DIFFERENTIATION III 



of yellow hyaloplasm, next this a layer of pigment, and 

 finally the unchanged yolk may be seen to come to the 

 surface at the vegetative (centrifugal) pole. 



The embryos have depigmented heads, or are headless 

 altogether, the anterior end being occupied by a swollen 

 vesicle. The degree of deformity depends on the length of 

 exposure to the centrifugal force. 



ii. When the eggs are centrifuged one and a half to three 

 hours after insemination for fifteen minutes or longer a fourth 

 white stratum appears, between the yellow hyaloplasm and 

 the pigment. The first division is usually unequal, and the 

 small blastomere segments more rapidly than the large one. 

 Many eggs die before gastrulation. Amongst those that do 

 develop half-embryos and headless monsters are frequent. 



3. The egg is centrifuged in the two-celled stage. 



A. Slowly for five hours. The first division is unequal, 

 and there is a tendency to meroblastic segmentation. 



B. Rapidly for from five to twenty -five minutes. The same 

 strata appear in each blastomere as in the second experiment, 

 and, with the longer exposures, there are abnormalities of 

 development, such as meroblastic segmentation. 



4. The eggs and anterior half -embryos are centrifuged 

 rapidly. After the completion of the third furrow the strata 

 appear in each of the eight cells. There is a tendency to 

 meroblastic segmentation, open blastopores, and anterior half- 

 embryos. These experiments make it perfectly clear that 

 derangement of the substances of the cytoplasm involves 

 abnormality in development. In the first place the yolk is 

 driven to the vegetative pole, and the distinction between the 

 protoplasmic and deutoplasmic regions of the egg thereby 

 increased. The yolk fails to segment, but the cytoplasm of 

 the animal hemisphere divides, and gives rise to a blastoderm. 

 The displacement of the yolk entails later on malformation of 

 the posterior end, normally developed near the vegetative pole, 

 with consequent persistence of the blastopore and restriction 

 of differentiation to the anterior half of the embryo. The 

 heads of these embryos are depigmented and the pigment is 

 therefore presumably inessential, but when the derangement 



