362 EDMUND B. WILSON 



the cysts develop progressively in a nearly continuous series from 

 one end to the other. All the cells in each cyst are nearly, but 

 often not quite, in the same stage. While the order of succession 

 is not demonstrated so accurately as in some objects (e.g., in 

 Batracoseps) it is placed practically beyond doubt by the study 

 of transitional conditions in cysts where slightly earlier and later 

 stages occur side by side. 



Throughout the whole complicated series of changes in the auto- 

 somes, the sex-chromosomes are at once recognizable at every 

 stage (save the very first) by their condensed and deep-staining 

 character. Lygaeus differs from Oncopeltus in the fact that the 

 Jf -chromosome always retains a rod-like form and is longitudin- 

 ally split at least as early as Stage /. In Oncopeltus both sex- 

 chromosomes remain in the form of rounded and apparently un- 

 divided chromosome-nucleoli up to Stage h, when they too assume 

 the form of short, longitudinally split rods. At the period of 

 synizesis the X-chromosome in Lygaeus shortens somewhat, 

 but at no time does it assume the rounded form characteristic 

 of Oncopeltus and many other forms. In this respect Lygaeus 

 bicrucis differs from L. turcicus, where the JT-chromosome has the 

 form of a much elongated and longitudinally split rod in the early 

 post-synaptic stages, but later contracts to a spheroidal form 

 (Wilson, '05 b) . These species of Lygaeus remove every doubt, 

 could such longer exist, of the identity of the chromatic 'nucleoli' 

 of the growth-period with a pair of chromosomes. 



2. The pre-synaptic period. Stages a to d 



The study of this period in these animals is of much interest in 

 relation to a series of questions, frequently raised in late years, 

 that are of the utmost importance for the theory of synapsis. 

 These are: (1) Are the leptotene-threads of this period chromo- 

 somes? (2) Is their number equal to that of the spermatogonial 

 chromosome-groups? (3) Can they be traced directly as individ- 

 uals to the anaphase-chromosomes of the last spermatogonial 

 division? As will be seen, the facts in the Hemiptera, in the dra- 

 gon-fly Anax, and in certain Orthoptera give good reason to 



