36 J. T. PATTERSON 



e. Origin and history of the asexual embryos 



I have already given an account of the history of the asexual 

 larvae (Patterson, '18), and shall quote rather freely from that 

 paper in this section. 



The appearance of non- viable, asexual larvae in polyembryonic 

 hymenoptera was first observed by Silvestri ('06) in Litomastix. 

 Briefly summarized, his account is as follows. The polygerm of 

 Litomastix, soon after the polar membrane is established, be- 

 gins to show differentiation into two distinct regions. The an- 

 terior part of the egg is made up of large and small cells, while 

 the posterior part is composed of small cells only. A constric- 

 tion develops in the polar membrane, which finally separates 

 these two regions. Silvestri calls the anterior region the massa 

 germinigena, and the posterior the massa monembrionale. The 

 posterior part subsequently differentiates into a single asexual 

 larva. In the course of further development, the massa germi- 

 nigena gives rise to a few secondary monembryonal masses, 

 which develop into asexual larvae, and to a large number of 

 other masses. This is accomplished by constrictions in the 

 polygerm. The masses continue to multiply by constrictions, 

 and from time to time may produce a few asexual embryos, but 

 a large majority of them develop into sexual embryos. In one 

 case Silvestri counted 100 asexual larvae arising from one egg; 

 in a second case he counted about 1700 sexual embryos and 220 

 asexual larvae. In structure the asexual larva differs from the 

 sexual larva in that it has no reproductive, respiratory, or cir- 

 culatory system, and no malpighian tubules. 



Silvestri has suggested that the asexual larvae may owe their 

 asexuality to the absence of germ cells. He bases his suggestion 

 on the fact that the so-called nucleolus, which in certain monem- 

 bryonic eggs seems to serve as a 'keimbahn-determinant,' is net 

 inherited by all of the embryonic cells. According to Silvestri 's 

 suggestion, an embryo arising from cells all of which are deficient 

 in nucleolar material would be asexual; while one receiving one 

 or more of these potential germ cells would be sexual. Aside 

 from the mechanical difficulty (to which I have already referred) 



