PLAXT-HOUSE ALEVRODES. 85 



broad end of the egg attaches it to the leaf and serves also as 

 the external part of the micropylar apparatus. The lumen of 

 the stalk is filled with protoplasm, which is in direct communi- 

 cation with a space in the end of the egg containing a mass of 

 granular protoplasm in which the nucleus and polar bodies are 

 situated. This space is separated from the yolk by a distinct 

 membrane. See Fig. 44. 



The yolk (Fig. 44) is composed of a great number of small, 

 nearly spherical bodies, each consisting of a vesicle containing a 

 viscid coagulable substance which stains deeply with haematoxy- 

 lin or carmine. The yolk granules are arranged so that there 

 are many comparatively large spaces which are filled with oily 

 fluid left between them. The oil globules are scattered evenly 

 through the substance of the yolk. Over the whole surface of 

 the yolk the peripheral protoplasmic layer is thin and incon- 

 spicuous. 



^Maturation of the egg takes place within the ovarian tubes of 

 the female. In the mature egg the polar bodies lie in the cham- 

 ber at the posterior end of the egg which contains the nucleus 

 and undifferentiated protoplasm. The eggs are fertilized in the 

 vagina of the female. The spermatozoon moves up through the 

 protoplasm contained in the stalk of the egg, while the female 

 pronucleus moves down and comes to lie at the entrance of the 

 stalk. Immediately after the act of fertilization, the protoplas- 

 mic contents of the stalk shrivel and dry up. \\ hen the two 

 pronuclei have fused, the resulting nucleus migrates back to the 

 center of the mass of protoplasm and at once begins active divi- 

 sion. The nuclei resulting from the first few divisions begin to 

 nnigrate toward the periphery of the egg, keeping an arrange- 

 ment conforming very nearly with the shape of the original 

 protoplasmic vesicle. The wall of the vesicle disappears as the 

 nuclei approach it. 



Each of the nuclei resulting from the frequent divisions is 

 surrounded by some of the protoplasm present in the vesicle. 

 As the nuclei migrate farther and farther from their point of 

 origin, it necessarily happens, on account of the location of the 

 parent nucleus, that they first reach the surface of the yolk in the 

 posterior part of the egg. When the nuclei reach the surface 

 of the yolk they begin to arrange themselves regularly in the 

 thin layer of protoplasm there present. The protoplasmic layer 



