176 



GENERAL EMBRYOLOGY 



sorption of water by the substance between the egg and the mem- 

 brane, a space of widely varying dimensions in different species, 

 is left between the egg and its membranes; this is the perivi- 

 telline space (Fig. 90). Within this space the egg is free to 



FIG. 91. Sections through the egg of the Tunicate, Cynthia partita. After 

 Conklin. X about 350. A. Ovarian egg fully formed. Germinal vesicle 

 surrounded by yolk bodies; peripheral layer of protoplasm containing test cells 

 and yellow granules (small circles). B. After extrusion of the test cells. Nuclear 

 membrane still intact with chromosomes at periphery of nucleus (germinal 

 vesicle). C. After laying (before fertilization, the egg remains in this condition 

 until fertilized). Chromosomes and granular substance, from which the spindle 

 is formed, lie in the center of the karyoplasm, now free in the cell, e, exoplasm 

 or cortical layer; g, granules of yellow pigment; n, egg nucleus or germinal vesicle; 

 t, nuclei of ingested test cells or follicle cells; y, yolk. 



move or rotate, although the superficial membrane may be 

 fixed to some foreign body. 



Frequently this phenomenon of membrane formation is but 



