DIFFERENTIATION, HEREDITY, SEX 267 



substances to the fundamental polarity and general organization 

 of the egg. In the opinion of some these substances are truly 

 to be regarded as the initial developmental differentiations of 

 the ovum. Each of these substances has a specific function 

 in development and leads to the formation of a certain tissue or 

 organ alone. Consequently these materials are known as 

 " organ-forming substances." Before cleavage they usually 

 assume a very definite symmetry of distribution, closely 

 related to that of the later developing organism, and during 

 cleavage they become distributed among certain specific groups 

 of cells whose lineage can be traced directly to the rudiments 

 of certain organs. 



Thus in the sgg of the Ascidian, Cynthia (Styela), fully 

 described by Conklin and one of the best examples of this type 

 of structure, at the close of the first cleavage there are five 

 regions of protoplasm, present in amounts roughly proportional 

 to the size of the parts to which they later give rise, and 

 distinguishable by the character of their granular contents 

 (Fig. 92). At the animal pole is a superficial region of com- 

 paratively clear protoplasm, the ectoplasm, from which the 

 ectoderm develops; in the vegetal pole there is a dark gray 

 region, the endoplasm, rich in yolk and later forming the 

 endoderm; the mesoderm is formed from a crescentic region, 

 the mesoplasm, located just below the equator, on the posterior 

 side; this is characterized by its content of yellow pigment, and 

 is divided into lighter and darker areas, forming respectively 

 the mesenchyme and the tail muscles (myoplasm); a light 

 gray crescent around the anterior border forms later the neural 

 plate and notochord (neuroplasm, chordaplasm). 



These substances are arranged symmetrically with reference 

 to the first cleavage plane and this corresponds also to the 

 median plane of the larva and adult, and thus from the first 

 separates right and left sides of the body. The second cleavage 

 plane, at right angles to the first, separates the yellow meso- 

 plasm from the light gray neuroplasm and chordaplasm, and 

 the third cleavage, horizontal, separates the clear ectoplasm 

 from the other substances. The later cell lineage of this form 



