1T4 HEREDITY AND ENVIRONMENT 



cesses, and yet they may also be the causes of 

 subsequent results. No thoughtful person 

 has ever maintained that chromosomes or any 

 other things in nature are autonomous, abso- 

 lute, uncaused causes. 



2. Cytoplasmic Correspondences. — How- 

 ever the most direct and the earliest recog- 

 nized correlations between the oosperm and 

 the developed animal are found in the polarity 

 and symmetry of the egg cytoplasm and of the 

 animal to which it gives rise. 



(a) Polarity. — In all eggs there is polar 

 differentiation, one pole, at which the matura- 

 tion divisions take place, being known as the 

 animal pole, and the opposite one being known 

 as the vegetative pole. The substance of the 

 egg in the vicinity of the animal pole usually 

 gives rise to the ectoderm, or outer cell layer 

 of the embryo; the portion of the egg sur- 

 rounding the vegetative pole usually becomes 

 the endoderm or inner cell layer. The axis 

 which connects these poles, the chief axis of 

 the egg, becomes the gastrular axis of the em- 

 bryo and in every great group of animals it 

 bears a constant relationship to the chief axis 



