182 THE COCKROACH: 



inexperienced embryologist will find it more profitable to 

 examine the eggs of Bees, of Aphides, or of such Diptera as lay 

 their eggs in water. 



The Cockroach is developed, like most animals, from fertilised 

 eggs.* The eggs of various animals differ much in size and 

 form, but always contain a formative plasma or egg- 

 protoplasm, a germinal vesicle (nucleus), and a germinal 

 spot (nucleolm). Besides these essential parts, eggs also always 

 contain a greater or less quantity of food-yolk, which serves 

 for the supply of the developing embryo. The quantity of 

 this yolk may be small, and its granules are then uniformly 

 dispersed through the egg-protoplasm ; or very considerable, 

 in which case the protoplasm and yolk become more or less 

 sharply defined. Eggs of the first kind are known as holoblastic, 

 those of the second kind as meroblastic, names suggested by the 

 complete or partial segmentation which these kinds of eggs 

 respectively undergo. When the food-yolk is very abundant it 

 does not at first (and in some cases does not at any time) 

 exhibit the phenomena of growth, such as cell-division. If, on 

 the other hand, the yolk is scanty and evenly dispersed through 

 the egg-protoplasm, the segmentation proceeds regularly and 

 completely. The eggs of Arthropoda, including those of the 

 Cockroach, are meroblastic. 



The eggs of the Cockroach (P. orientals) are enclosed (see 

 p. 23) sixteen together in stout capsules of horny consistence. 

 They are adapted to the form of the capsule, laterally com- 

 pressed, convex on the outer, and concave on the inner side. 

 The ventral surface of the embryo lies towards the inner, 

 concave surface of the egg. Each egg is provided with a very 

 thin brownish shell (chorion), whose surface is ornamented with 

 small six-sided projections. In young eggs, still enclosed within 

 the ovary, the nucleus (germinal vesicle) and nucleolus (germinal 

 spot) can be plainly seen, but by the time they are ready for 

 deposition within the capsule, so large a quantity of food-yolk, 

 at first finely afterwards coarsely granular, accumulates 

 within them, that the germinal vesicle and spot cease to be 

 visible. 



* Fertilisation consists essentially in the union of an egg-nucleus (female nucleus) 

 with a sperm-nucleus (male nucleus). From this union the first segmentation-nucleus 

 is derived. 



