648 



ZOOLOGY 



SECT. 



seg 



blast 



Development. — The segmentation is usually of a type already 

 referred to (p. 597) as very common among the Crustacea, viz., 

 superficial segmentation. The actual segmentation (Fig. 527) 

 has chiefly been observed in the case of certain Insects with very 

 little yolk ; but there can be very little doubt that in ordinary 



forms with abundant yolk the 

 process is in essence the same. 

 The segmentation-nucleus, ori- 

 ginally situated near the middle 

 of the ovum, divides into a 

 number of nuclei, and most or 

 all of these migrate towards 

 the surface, and arrange them- 

 selves in the form of a sphere 

 almost parallel with the latter ; 

 eventually they reach the sur- 

 face and coalesce with the peri- 

 pheral protoplasm, which then 

 becomes divided into cell-areas 

 corresponding with the nuclei. 

 The layer of cells thus formed 

 constitutes the blastoderm. 

 This thickens along one side 

 to form the ventral plate, as 

 already described in the case of 

 the Cockroach (p. 629), and the 

 changes which this structure 

 undergoes, together with the 

 mede of formation of the ap- 

 pendages, are similar in most 

 members of the class, except 

 that in most Insects the forma- 

 tion of the lower layers is 

 associated with a more or less 

 distinct invagination (Fig. 528). 

 The same holds good of the 

 formation of the amnion and 

 the development of the meso- 

 derm and endoderm. In some 

 cases there is developed be- 

 tween the serosa and the true 

 amnion a space filled with yolk, and the ventral plate appears 

 sunk within the yolk. The nervous system is developed from 

 the ectoderm in the manner indicated in the account of the 

 Cockroach (p. 631). The tracheal system is derived from a 

 series of pairs of segmental ly arranged ectodermal involutions 

 (Fig. 530, st). 





Hast 



yk.u 



Fib. 527. — A — D, successive stages in the seg- 

 mentation of the ovum of an Insect blast. 

 blastoderm ; peri, peripheral protoplasm ; 

 seft. segmentation-cells ; yk. yolk ; ykc. yolk- 

 cells. (From Korschelt and Heider, after 

 Blochmann.) 



