96 



Mr. L. Doiicaster. 



[Sept. 21, 



rom the primitive ova at the top of the egg-tube. The tube is thus monili- 

 form, and the swellings contain alternately a developing egg and a mass of 

 cells like primitive ova (fig. 20). 



As the eggs develop they enlarge greatly by the deposition of yolk, and the 

 nucleus appears as a relatively small vesicle, lying at one side. When ripe, 

 the egg is prolonged at the front end into a hollow stalk with a small 

 vesicle at the free end ; the stalk is said to be held by the ovipositor as the 

 egg is thrust into the hole bored in the leaf when the egg is being laid. The 

 stalk thus partly blocks the opening, and in the spring eggs, which are 

 generally somewhat compressed by the bud-scales, some of the yolk of the egg 

 is frequently forced into the cavity of the stalk. 



After one of the primitive ova has definitely begun to develop into an egg, 

 as shown by the deposition of yolk, no division of the nucleus appears to take 

 place until it is laid. In the follicle-cells which surround the eggs mitoses 

 are not uncommon, and show the same number of chromosomes as in other 

 somatic nuclei, viz., about 20. 



In the females of the spring (agamic) generation, I have not observed th& 

 oogonia. In the pupse the development does not differ materially from that 

 of the summer generation. The egg-tubes are very similar, and the eggs 

 when laid differ chiefly in being slightly larger. Since the pupaj develop 

 very slowly, mitoses are more difficult to find, but the chromosome gi-oups 

 from an egg-follicle, represented in fig. 3, show that the somatic number is 

 similar to that in the summer pupse. Fig. 21 «, b, c, shows three figures of 

 an egg-nucleus in which about 20 chromosomes appear to be visible, a and 

 b being drawn from the same section at different levels, and c from the next 

 section of the series. 



Maturation and Fertilisation of the Egg. — Slimmer Generation. 

 The eggs laid by the flies of the summer generation in the early part of 

 June are sunk in the tissue of the underside of young oak leaves, and the 

 stalk of the egg commonly projects somewhat from the hole made by tlie 

 ovipositor. These eggs are fertilised, and the large sperniatazoon may ahnost 

 always be found near the edge in sections of eggs preserved within about an 

 hour and a half from the time of laying. The spermatozoon, after entering 

 the egg, and while being converted into the male pronucleus, generally lies 

 at the opposite side of the egg, or at least some distance removed, from the 

 egg-nucleus, which during the same period is undergoing its maturation pro- 

 cesses. During the first hour and a half after the egg is laid the nucleus is 

 found near the edge ; its position is somewhat variable, but it is commonly 

 about midway between tlie ends of the egg (1-late 2, fig. 22). 



