THE DEVELOPMENT OF AN INSECT 123 



August, and September. Draw everything that can be clearly 

 seen. 



It is not possible to give precise information as to the 

 age of the various stages. Development proceeds at very 

 different rates in warm and cold weather. The whole time 

 required is about six days, but varies from four to seven. 



Stage 1. (The undeveloped egg). It is not easy to get 

 eggs immediately after laying. I have found it necessary 

 to capture a female in the act of fertilisation, and imprison 

 her in a bell-glass with a cup of water. At this time, and 

 more or less for some hours after, the following details are 

 visible. The egg is contained in a transparent oval egg-shell, 

 which is flattened along one side. Within the egg-shell is 

 a transparent vitelline membrane, hardly to be distinguished 

 at present, but becoming plainer in the course of development. 

 A mass of yellow, granular yolk occupies the centre of the 

 ■egg, while the outermost part is clear, and shows no structure. 

 The formation of polar bodies and pronuclei can be studied 

 by those who have experience of embryological methods. 



Stage 2. About two hours after egg-laying, segmentation 

 begins at the hinder pole. A few large nuclei appear in 

 successive pairs at the surface of the egg, the protoplasm 

 gathers round each, and forms constricted masses, which after- 

 wards bud off, complete themselves, and divide into four and 

 •eight. After this, the nuclei divide without separating, and 

 form eight large cells, with four nuclei apiece. These are 

 the sexual germ-cells. At this time the yolk contains several 

 scattered nuclei, which can only be demonstrated by refined 

 methods. They multiply with great rapidity, travel towards 

 the surface of the egg, and arrange themselves side by side 

 in a transparent layer, which completely lines the egg-shell. 

 This layer is the blastoderm; it gives rise to the whole 

 body, and also to certain temporary structures, which are 

 afterwards cast off'. The blastoderm appears very suddenly, 

 and the act of formation is hard to observe. Within the 

 blastoderm a clear zone appears, and a regular layer of nuclei 

 forms in it, so that the blastoderm soon becomes two-layered. 



Stage 3. It is only by much attention that the events 

 already described can be actually observed, but the subsequent 

 stages present far less difficulty. Only the most essential 

 features are described here. The cellular layer, or blastoderm, 



