54 SUMMARY OF CURRENT RESEARCHES RELATING TO 



arc to be found in the outer layer of protoplasm, and two in the middle 

 of the yolk ; the blastoderm docs not yet form a continuous layer, but 

 is here and there broken through by the subjacent yolk. Polar 

 differentiation now commences, the cells at one pole appearing 

 larger than those at the other, so that the egg becomes pyriform in 

 shape ; this becomes more marked in succeeding stages. Cleavage 

 is effected very rapidly. After a detailed account of the whole pro- 

 cess of development, the author passes to (2) the development of the 

 oviparous females and of the males ; the chief difference between the 

 two sorts lies in the history of the generative organs. When their 

 ecdyses are complete, the males fertilize the females, which produce 

 eggs of proportionately very large size, brown in colour, filled with 

 small yolk-granules and fat-drops. As there are four times as many 

 females as males, each male must fertilize several females and live 

 for a longer period than they ; the egg, after the winter, produces an 

 apterous viviparous female (of the first generation), which contains 

 in its ovarian tubes a larger number of chambers than do the suc- 

 ceeding apterous generations. (3) The development of the viviparous 

 female from the winter egg is next discussed, but here the author's 

 opportunities were not so great as for the previous conditions ; from 

 what he has been able to see and from what Balbiani has taught us, 

 he is inclined to ascribe such differences as there are to the possession 

 of a large amount of nutrient yolk. 



The process of formation of the blastoderm in insects generally 

 appears to be of the following character : the mature ovum has a 

 peripheral layer of protoplasm, within which are yolk-granules and 

 fat-drops ; the difference between proto- and deuto-plasm is not so 

 sharply marked ; in the yolk, and towards the pole which later on is 

 the animal pole, there is the germinal vesicle, the nuclear membrane 

 of which becomes absorbed, and the nuclear corpuscles undergo 

 changes which are invisible in the fresh egg. The vesicle divides, 

 as do its descendants, imtil there are a large number of nuclei, some 

 of which make their way into the peripheral layer of protoplasm to 

 form the blastoderm, while the rest go to form yolk-cells. In the 

 periphery cells soon become differentiated, but in the yolk they are 

 only formed slowly, or not at all ; this is because of the smaller 

 quantity of protoplasm which is there present. The gastrula is 

 formed by emboly. Insects are distinguished by a peculiar arrange- 

 ment of the formative and nutrient yolk ; and, on account of the size of 

 the egg, its contents cannot divide simultaneously with the germinal 

 vesicle, so that the mode of change is " endovitelline." Before the 

 formation of the blastoderm, the egg must be looked upon as being in 

 a syncytial stage. The mechanical effect of gastrulation is produced 

 by the passage of the nuclei into the peripheral layer. The peculiar 

 mode of development of the insect ovum is to be explained by its 

 size, by the position of the germinal vesicle, and by the fact that the 

 whole egg is inclosed in a layer of protoplasm. 



After discussing the germinal bands and the embryonic membranes 

 which have received attention from various preceding writers, the 

 author takes up the germinal layers ; in insects, as in vertebrates, the 



