120 GENERAL EMBRYOLOGY 



cells have the arrangement of an epithelium; they may form 

 either a single layered, simple epithelium or in other cases a 

 many layered stratified epithelium (Fig. 43). They not only 

 provide for the nutrition of the enlarging ovum, with ^vhich 

 they are frequently connected by definite intercellular proto- 

 plasmic strands, but toward the close of the growth period of 



the egg they may become secre- 

 tory and form certain egg envel- 

 opes of the secondary type, i.e., 

 chorionic. We have already men- 

 tioned how the micropyle is 

 formed through the chorion and 

 vitelline membrane by the inser- 

 tion of one of these follicle cells 

 with a long process, preventing the 

 membranes from forming at that 

 point. When the eggs are fully 

 formed and ready to be laid the 

 follicle ruptures 'allowing the eggs 

 to escape freely. Often there de- 

 velops in the follicle a definite 

 region along which it bursts; this 

 weakened region is called the 

 cicatrix (e.g., common fowl). In 

 a few instances some of the follicle 

 cells are actually taken into the 

 egg and absorbed, much as in the case of those ova which in- 

 gest adjacent interstitial cells. This is the case in many of 

 the Tunicata where some of the so-called "test-cells" lose 

 their cell outlines and are directly taken into the cytoplasm 

 of the egg; their nuclei remain distinct for some time (Fig. 67). 

 The remaining test cells then form a distinct follicle outside 

 the whole structure. These nuclei no longer function as 

 nuclei of course; as the growth of the egg is completed they 

 are extruded again, along with a portion of the superficial pro- 

 toplasm forming then a thick vitelline membrane resem- 

 bling a chorion and often so called. 



FIG. 67. Section through the 

 egg of the Tunicate, Cynthia 

 partita. After Conklin. In the 

 periphery of the egg are the nuclei 

 of the test cells which have been 

 ingested by the ovum (for their 

 later history see Fig. 91). X 357. 

 e, exoplasm or cortical layer; n, 

 egg nucleus or germinal vesicle, 

 with large nucleolus; t, nuclei of 

 test cells or follicle cells; y, yolk. 



