BIOLOGY OF PARACOPIDOSOMOPSIS. 303 



V. THE ASEXUAL LARVAE. 



Asexual larvae were first described by Silvestri for Litomastix. 

 According to this investigator, the egg of Litomastix produces a 

 thousand or more sexual larvae and a variable number of asexual 

 larvae, which are characterized by the absence of certain organs, 

 and especially by the absence of the reproductive system. These 

 larvae die without undergoing metamorphosis. Exactly similar 

 larvae develop from the egg of Paracopidosomopsis. The details 

 of their development have not as yet been studied, so that I 

 cannot say whether they appear in every egg or not. These 

 curious larvae develop both in the fertilized and in the unfertilized 

 egg, even though the host egg be rigidly protected from all other 

 parasites. This shows that the asexual larvae do not come from 

 the egg of another parasitic species, as has been suggested as an 

 explanation for their appearance among the larvae of Litomastix 

 (Wheeler, '10). An account of their development will be made 

 the subject of a subsequent paper. Here, we are concerned with 

 the question of the underlying cause of their production. 



Silvestri suggests that these asexual larvae may owe their 

 asexuality to the absence of germ cells. It will be recalled that 

 he described for the polyembryonic egg a so-called nucleolus 

 (nucleolo), which lies at the larger or posterior end of the elon- 

 gated, bottle-shaped egg. In the early cleavage stages of 

 Litomastix the nucleolus becomes included in a single blasto- 

 mere, resulting in the retardation of the subsequent divisions 

 of this cell. The nucleolus later breaks down and becomes 

 scattered evenly throughout the cytoplasm. Silvestri was 

 unable to follow the history of this particular cell beyond two 

 divisions, but he believes that it gives rise to all the germ cells 

 of the sexual embryos. His conclusion has been strengthened 

 by a subsequent study of the eggs of two monembryonic parasites. 

 The nucleolus in the egg of these species becomes distributed to 

 the germ cells alone, and thus serves as a " keimbahn-determi- 

 nant" (Hegner). 



According to Silvestri's suggestion the asexual larvae of Lito- 

 mastix arise from the polygerm, but fail to receive germ cells or 

 descendants of the single blastomere which inherited the nucle- 

 olus. It may be that this suggestion of Silvestri offers the correct 



