CLEAVAGE AND THE FORMATION OF THE BLASTODERM. 263 



with a delicate chromatine network. During the process of maturation 

 it shifts to the surface of the egg, there a spindle is formed, and the 

 nucleus undergoes division. In this way the first, and afterwards 

 the second, polar body is formed (Blochmaxx, No. 5). 



The position of the polar spindle varies in the different groups of the Insecta. 

 In some (Picris) it lies directly at the anterior pole of the egg, but in most 

 Insects it is formed nearer the middle of the long axis of the egg. BLOCHMANN 

 (No. 5) found it in Blatta in the middle of the dorsal surface, while in Musca it 

 occurs at about one-third to one-fourth of the whole length behind the anterior 

 pole on the concave (dorsal) side of the egg (Fig. 129, r). In the Formicidac it 

 also lies laterally, but near the anterior pole, while in the Aphidae it is situated 

 at the middle of one of the lateral surfaces of the egg. In Hydrophilus (Heidek) 

 it lies somewhat behind the mid-lateral region. 



"o* 



2. Cleavage and the Formation of the Blastoderm. 



The type of cleavage universally prevalent among the Insecta* is 

 the pure superficial type so common among the Arthropoda (Vol. ii., 

 (p. 113). The first cleavage-nucleus (produced in the fertilised egg 

 by the union of the male and female pronuclei) shifts inward and 

 increases by indirect nuclear division (Figs. 131 A and 175 A, B, 

 C, f). The formation by division of the numerous cleavage-nuclei 

 from the first has only been directly observed in those eggs that 

 are poor in yolk (Ajrfiidae, Gecidomyia, Cynipidae). But it can 

 hardly be doubted that in the larger yolk-bearing eggs of other 

 Insects the numerous cleavage-nuclei which are found distributed 

 throughout the egg soon after it has been laid are actually pro- 

 duced from the first cleavage-nucleus by nuclear division. These 

 numerous nuclei, with the star-like areas of protoplasm which sur- 

 round them, represent the formative elements of the blastoderm. 

 Tichomiroff has, however, conjectured in the case of the egg of 

 Bombyx, and Hexkixg (Xo. 39) has more definitely maintained in 

 the case of Musca that these nuclei in their protoplasmic islands 

 distributed throughout the yolk-masses have been produced by free 

 formation of nuclei. t But this view appears to us altogether un- 

 tenable. It is contradicted by the observations of Blochmaxx 

 (Xo. 5), according to which all the cleavage-nuclei in Musca undergo 



* Uljanin (Xo. 83) believes that total and equal cleavage occurs in the 

 Poduridae. It appears, however, from the researches of Lemoixe that even 

 here cleavage is superficial, and the same conclusion is arrived at by Gkassi 

 (Xo. 33) from the condition of the food-yolk in the later stages in Japyx. 

 [According to Henneiuy (No. XII.), cleavage is total in the eggs of Smicra, 

 one of the Cha/cididae. This is obviously a derived condition, as these are 

 parasitic forms and the egg is nourished at the expense of the host. — Ed.] 



t [Henkinu (No. XI.) has since modified this statement. See also footnote 

 p. 167.— Ed.] 



