130 



ENTOMOLOGY 



sperm nucleus unites with the egg-nucleus to form what is known as 

 the segmentation nucleus. Through this union of nuclear substances 

 the qualities of the two parents are combined in the offspring. 



Needless to say, the minute 

 details of the process of fertili- 

 zation are of the highest bio- 

 logical importance. 



Blastoderm. In an arthro- 

 pod ovum the yolk occupies a 

 central position (centrolecithal 

 type), being enclosed in a thin 

 layer of protoplasm. From the 

 segmentation nucleus just men- 

 tioned are derived many nuclei, 

 some of which migrate out- 

 ward with their attendant pro- 



V ^^ygg^^ toplasm to form with the 



FIG. 191. Equatorial section of egg of a beetle, original peripheral protoplasm 



Clytra Iceviuscula. b, blastoderm -g, germ band; .. 1111 j_i 



y, yolk granule; yc, yolk cell. After LECAILLON. a Continuous Cellular layer, the 



blastoderm (Fig. 191). 



Germ Band. The blastoderm, at first of uniform thickness, be- 

 comes thicker in one region, by cell multiplication, forming the germ 

 band (primitive streak, etc.); this appears in surface view as an oval 

 or elongate area, denser than the remaining blastoderm, with which 

 it is, of course, continuous. 



Gastrulation. The germ band next infolds along the median line, 



appearing in cross- 

 section as in Fig. 192 ; 

 the two lips of the 

 median groove close to- 

 gether over the invagi- 

 nated portion and 

 form an outer layer, or 

 ectoderm (Fig. 193), 

 while the invagi- 

 nated portion spreads 



out as an inner layer, which is destined to form two layers, known 

 respectively as entoderm and mesoderm. This formation of two primary 

 germ layers by invagination or otherwise is termed gastrulation; it is 

 an important stage in the development of all eggs, and among insects 

 several variations of the process occur. 





FIG. 192. Transverse section of germ band of Clytra at 

 gastrulation. g, germ band; , inner layer. After LECAILLON. 



