DEVELOPMENT 



FIG. 189. Equatorial section of egg of a 

 beetle, Clytrdlcrduscula. b, blastoderm; g, germ 

 band; y, yolk granule; yc, yolk cell. After 

 LECAILLON. 



known as the segmentation nucleus. Through this union of nuclear sub- 

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

 Needless to say, the minute details of the process of fertilization are of 

 the highest biological impor- 

 tance. 



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 outward 

 with their attendant protoplasm 

 to form with the original peri- 

 pheral protoplasm a continuous 

 cellular layer, iheblastoderm (Fig. 

 189). 



Germ Band. The blasto- 

 derm, at first of uniform thick- 

 ness, becomes thicker in one re- 

 gion, 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. 190; 

 the two lips of the 

 median groove close 

 together over the in- 

 vaginated portion and 

 form an outer layer, 

 or ectoderm (Fig. 191), 

 while the invaginated 

 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. 190. Transverse section of germ band of Clytra at gas- 

 trulation. g, germ band; i, inner layer. After LECAILLON. 



