146 EMBRYOLOGY 



CHAP 



from the minute formative portions of the egg and the changes 

 that take place in it have been to some extent studied, and 

 Kowalewsky, Dohrn/ Woodworth,^ and others have given some 

 particulars about them. The early changes in the formative 

 parts of the mature egg have been observed by Henking in 

 several Insects, and particularly in Pyrrhocoris, his observations 

 being of considerable interest. When the egg is in the 

 ovary and before it is quite mature, at the time, in fact, 

 when it is receiving nutriment from ovarian cells, it contains 

 a germinal vesicle including a germinal spot, but when the 

 ^gg is mature the germinal vesicle has disappeared, and there 

 exists in its place at one portion of the periphery of the egg- 

 contents a cluster of minute bodies called chromosomes by Henking, 

 whom we shall follow in briefly describing their changes. The 



group divides into two, each of 

 which is arranged in a rod or 

 spindle - like manner, and may 

 then be called a directive rod 

 or spindle. The outer of these 

 two groups travels quite to the 

 periphery of the egg, and there 

 with some adjacent matter is 

 extruded quite outside the egg- 

 contents (not outside the egg- 

 coverings), being in its aug- 

 mented form called a polar or 

 directive body. While this is 

 going on the second directive 



Fig 79.-Shovving the two extruded polar gpindle itself divides into twO 



bodies Pi, P.2 now nearly fused and re- ^ 



included, and the formation of the grOUpS, the OUtcr of which is 



spindle by junction of the male and ^i^ PYtrnriprl in thp manTiPv 



female pronuclei. (After Heukiug.) ^^^^ extiuaeci in tne mannei 



we have already described in 

 the case of the first polar body, thus completing the extrusion of 

 two directive bodies. The essential parts of the bodies that 

 are successively formed during these processes are the aggregates, 

 called chromosomes ; the number of these chromosomes appears 

 to be constant in each species ; their movements and dispositions 

 are of a very interesting character, the systems they form in 



^ Zeitsclir. wiss. Zool. xxvi. 1876, p. 115. 

 Scudder, Butterflies of New England, i. 1889, p. 



I 



