264 



INSECTA. 



division simultaneously (Fig. 130 



A 



A 



jXx, 



OR 



. ■ oH^o^L " -eta, - 



*««*:* 







Fig. 130. — Stages of blastoderm-formation 

 in Musca (Callipkora) vomitoria (after 

 Blochmann). The drawings represent parts 

 of sections through four eggs. A, the 

 nuclei of the cleavage-cells have become 

 arranged parallel to the surface of the egg. 

 ]:, the cleavage-cells fuse with the peripheral 

 protoplasm. C, the surface becomes in- 

 dented by furrows ; all the nuclei of the 

 blastoderm-cells are shown in the act of 

 division. D, the blastoderm-cells in the 

 form of long cylindrical epitheloid cells. 

 6, peripheral protoplasm ; bz, blastoderm- 

 cells; d, food-yolk; dz, yolk-cells; fz, so- 

 called cleavage - cells ; i, inner peripheral 

 protoplasm. 



O), which indicates that they 

 represent a generation of descen- 

 dants of the first cleavage-nucleus, 

 all being of the same age ; and it 

 is further disproved by the direct 

 observations above mentioned as 

 being made on some small eggs. 



According to Weismanx (No 89) 

 in lihodites and Biorhiza aptera (Ci/ni- 

 pidae) the first cleavage-nucleus divides 

 at first into two nuclei which shift 

 apart in the direction of the longi- 

 tudinal axis of the egg, and, according 

 to their positions, are known as the 

 anterior and posterior "pole nuclei."* 

 While the anterior nucleus remains in- 

 active for some time, the posterior, by 

 a kind of budding (?), gives rise to 

 numerous nuclei, which take part in 

 the formation of the blastoderm. The 

 anterior nucleus, on the contrary, after 

 the completion of the blastoderm, is 

 said to produce by division the nuclei 

 of the so-called inner germ -cells or 

 yolk-cells. 



The process of the formation 



of the blastoderm in larger eggs 



O Oct 



rich in yolk was first followed 

 in detail by Bobretzky (No. 6) 

 and Graber (No. 149), with the 

 help of sections. More recently 

 Blochmann (No. 5) has made 

 investigations on the Muscidae 

 with which the statements of 

 Heider (No. 38) concerning Hij- 

 drophilus agree. The cleavage- 

 nuclei at first lie at the centre of 

 the egg, more or less in the longi- 

 tudinal axis (Fig. 131 A). Each 

 of these nuclei (/) is surrounded 

 by a star-like mass of protoplasm, 

 and the whole is therefore not 



* [This term is used only to describe the position of these two first cleavage- 

 nuclei in the elongate egg, and has no connection with the nuclei of the polar 

 bodies, or with the "pole-cells," p. 352.— Ed.] 



