244 ROBERT W. HEGNER. 



and were allowed to develop until n A.M. June 25. The pos- 

 terior end of sixty-five of them was then killed with a hot needle. 

 The four controls hatched on June 29. The eggs when operated 

 upon were in a stage like that shown in Fig. 2. One of the 

 operated eggs hatched on June 30 (L.D. on B6), several were 

 hatching on the following day, and a number of others were ready 

 to hatch at that time. The one that hatched and three of those 

 ready to hatch were examined and then sectioned and stained. 

 Superficially they resembled the larva shown in Fig. 7, the only 

 difference being the absence of the last two posterior segments 

 which are indicated by the letter x in Fig. 7; one possessed all 

 but the last segment. The sections showed that none of these 

 larvae contained germ cells. 



It is evident from these experiments that the primordial germ 

 cells were killed by the operation and no new ones were produced 

 by the developing embryos. This is, I believe, the earliest 

 stage at which surgical castration has been performed among 

 the Insecta. The influence of this operation upon secondary 

 sexual characters could not be determined, since none of these 

 characters make their appearance in the larvae. 



4. KILLING PARTS OF FRESHLY LAID EGGS. 



The experiments described under heading 2, "Killing the Germ 

 Cell Determinants," might be included in this part of the paper, 

 but they were considered of sufficient importance to warrant a 

 special account. In performing the experiments in series L.D. 

 09 (Table I.) it was found impossible to regulate with any degree 

 of exactness the amount of the egg killed, and, since it was ab- 

 solutely necessary that all of the germ cell determinants be killed, 

 a larger portion of the egg was killed than desired. Some of these 

 eggs, however, when allowed to continue their development, 

 provided data with regard to the effects of killing a large part of 

 the posterior end of freshly laid eggs. 



Fig. II was drawn from an egg from series L.D. 09 64. The 

 portion killed by the operation is labelled k; the material that 

 remained alive produced the head (/?) and part of the thorax (/) 

 of an embryo. These parts resemble the corresponding parts 

 of a normal embryo at a like age (see Fig. 6). Apparently that 



