610 



INVEETEBEATA 



CHAr. 



Fig. 442. — The egg of Cynthia partita 

 before and during fertilization. 

 (After Conkliu.) 



[The yellow cytoplasm is indicated by 

 dotting the dark-blue cytoplasm by dark 

 shading.] 



A, egg before fertilization with large 

 germinal vesicle. B, egg immediately after 

 fertilization, showing the down-streaming 

 of the yellow cytioplasm and the outflow of 

 the contents of the germinal vesicle. C, 

 egg a little later, showing the aggregation 

 of the yellow cytoplasm to form the yellow 

 crescent at the posterior side of the egg. 

 ect.cy, clear cytoplasm which will give rise 

 to ectoderm; e.m, egg - membrane ; g.v, 

 germinal vesicle ; t.c, test cells ; y.&r^ yellow 

 crescent. 



acid. Such material eau be stained 

 with haematoxylin followed by eosin 

 dissolved in oil of cloves, on the slide. 

 Conklin commenced his study of 

 the development with the ovarian egg. 

 In the young oocyte there is no yolk, 

 and the nucleus occupies a nearly 

 central position in the egg. On the 

 one side of the nucleus is a rounded 

 mass which stains deeply with eosin. 

 This Conklin considers to be the 

 " attraction sphere," which functioned 

 in the cell division which gave rise 

 to the oocyte; it now functions as 

 yolk -nucleus, i.e. the deposition of 

 yolk -granules takes place round it 

 as a centre. A layer of peripheral 

 cytoplasm remains tree from yolk ; in 

 tlais, however, yellow pigment granules 

 are deposited. The yolk spherules are 

 of a slaty-grey colour. There is an 

 enormous vesicular nucleus (germinal 

 vesicle), with a large nucleolus, and 

 near the nuclear membrane a few 

 chromatin granules. 



As the oocyte grows it is surrounded 

 by the so-called test cells. These are 

 simply follicle cells or rudimentary 

 ova ; round the nucleus of each there 

 is the same collection of yellow 

 granules as are found in the peripheral 

 layer of the ovum. These " test cells " 

 become actually embedded in the 

 peripheral layer of the oocyte whilst 

 it is growing, but when it is ripe and 

 discharged from the ovary the test 

 cells are expelled from the cytoplasm, 

 and a chorion is formed. Between 

 the chorion and the egg there is a 

 space formed by the shrinkage of the 

 latter, the perivitelhne space. In. this 

 the expelled test cells he. 



After laying, the egg begins to 

 undergo maturation changes. The 

 nuclear membrane dissolves and so 

 does the large nucleolus. The clear 

 nuclear sap flows upiva^'ds and forms 

 a cap of clear cytoplasm, lying at 



