58 



CHEMODIFFERENTIATION 



In the protoplasm of newly laid eggs of this group, three differ- 

 ent substances can be distinguished because of their different 

 pigmentation. At the animal pole, we find the ectoplasm. This 

 has originated in the course of maturation from the nuclear 

 sap of the oocyte nucleus; on the disappearance of the nuclear 

 membrane, this sap mixes with the surrounding e^g proto- 

 plasm. A thin layer of mesoplasm lies at the surface of the egg, 

 and the rest of it comprises the endoplasm (Fig. 22a). Soon 



Fig. 22. The egg of an ascidian, before (a) and after (b) fertilisa- 

 tion, ect: ectoplasm; end: endoplasm; mes: mesoplasm; ch.n: chor- 

 doneuroplasm. After Conklin. 



after fertilisation, complicated streaming movements of these 

 substances can be observed. Ultimately the following distribu- 

 tion is reached: the ectoplasm occupies the animal side, and 

 the endoplasm the vegetative side of the egg; the mesoplasm 

 has accumulated at the ventral side, near the equator, forming 

 the so-called "yellow crescent". Meanwhile a fourth substance, 

 the chordoneuroplasm has appeared at the dorsal side of the 

 egg, also near the equator, where it forms the so-called "grey 

 crescent" (Fig. 22b). Consequently, the egg now has a clear-cut 

 bilateral symmetry. In the course of cleavage, the various 

 cytoplasmic substances become localized in different cells. In 

 further development they will give rise to different organs and 

 tissues of the embryo. The skin is produced by the cells contain- 

 ing the ectoplasm; those with the endoplasm will form the gut. 

 The cells of the yellow crescent, which contain the mesoplasm, 

 will supply musculature and connective tissue, and the chordo- 

 neuroplasmic cells of the grey crescent will develop partly into 

 notochord and partly into central nervous system. 



