THE INSECTA 165 



an outer chitinous chorion secreted by the follicle. As the 

 ova are passed out of the vulva they are fertilised by sperma- 

 tozoa discharged from the spermatheca, and are packed sixteen 

 together, like cigars in a cigar case, in a horny egg-case formed 

 by the secretion of the colleterial glands. The female carries 

 the egg-case for some days in the genital pouch, and eventually 

 deposits it in some sheltered place, preferably near some con- 

 venient supply of food for the young when hatched out. 



The yolk which forms the bulk of the egg is of so refractory 

 a nature that the study of the earlier phases of the development 

 of the cockroach is a matter of great difficulty, and the intri- 

 cacies of insect embryology are such that it would be out of 

 place to attempt to describe them in detail in this place. The 

 earlier stages may be briefly summed up as follows : The 

 nucleus of the fertilised ovum lies at first in the centre of the 

 yolk mass. It divides repeatedly, without any corresponding 

 division of the yolk, till some sixty to eighty nuclei are formed. 

 These nuclei migrate to the surface of the yolk, and continue 

 to divide until a stage is reached in which there is a central 

 yolk mass surrounded by a sheet of protoplasm containing 

 numerous nuclei. Eventually the protoplasm is divided into 

 cells, and the yolk is then invested by a sheet of tissue one cell 

 thick, called the blastoderm. On the concave side of the egg 

 the blastoderm cells become columnar and form an elongated 

 band known as the ventral plate, elsewhere they are thin and 

 flat like a pavement epithelium (fig. 41, A). The ventral plate 

 is at first one cell thick, but a longitudinal groove is soon formed 

 at its hinder end and the cells at the bottom of the groove 

 divide rapidly by tangential divisions, giving rise to a deeper 

 layer of cells which gradually spreads forwards under the outer 

 layer. The latter may now be called the epiblast, the deeper 

 layer will give rise to both mesoblast and hypoblast. As growth 

 proceeds the lateral parts of the deeper layer extend forwards 

 in the form of two bands towards the anterior part of the 

 ventral plate, and there the bands diverge slightly from one 

 another and form a pair of swellings known as the cephalic 

 plates, the bands themselves are the mesoblast bands. The 

 middle part of the deeper layer becomes separated from the 

 mesoblast bands and grows forward more slowly. Eventually 

 it extends to the front end of the ventral plate and bifurcates 

 above the cephalic plates, taking up a position between the 



