THE INSECTS 123 



of the Space available to it. Between these two extremes there are various 

 intermediate types of eggs, of which those of the Hymenoptera and 

 Coleoptera are the best knov^ai. 



An experimental study of development in the related group of Arach- 

 nids has recently been published (Holm 1952). 



2. Experimental analysis of some types of insect development^ 



It is the indeterminate type, with its capacities for regulation, which 

 allows us most easily to gain an insight into the epigenetic system of the 

 insects. We shall therefore begin by considering a typical representative 

 of this type, the dragon-fly (Odonata) Platycnemis pennipes (Seidel 1929, 

 1936). 



The cleavage of the nuclei, their repulsion from one another, and the 

 eventual formation of a blastoderm covering the whole surface pro- 

 ceed in an absolutely typical manner (Fig. 8.2). A germ-band then 

 appears, in the form of a region of the blastoderm which is thicker and 

 contains a higher concentration of nuclei. It at first shows some signs of 

 doubleness, but soon shortens somewhat and becomes a single area except 

 in its most anterior part, where there are two lobes which will eventually 

 develop into the head. The next stages of differentiation — that is, the 

 further thickening of the germ-band, the folding inwards of the lower 

 layer, the formation of transverse segments and the appearance of the 

 appendages — all begin in a region which lies a short distance posterior 

 to the head, in an area which later becomes the anterior thorax, and they 

 spread both anteriorly and posteriorly from there until they affect the 

 whole germ. This region, where development is visibly most advanced, 

 is knovwi as the Differentiation Centre ; we shall see that it has important 

 physiological functions as well as being morphologically in the lead. 

 During the development of the embryo, a considerable blastokinesis 

 occurs, the whole germ being at first folded deep into the yolk, and then 

 emerging again, at the same time twisting around its longitudinal axis, 

 so that it eventually lies with its dorsal side against the same part of the 

 egg membrane as was originally in contact ymxh. its ventral face. The 

 dorsal wall is not actually completed until after these blastokinetic move- 

 ments have ceased. 



The early stages of the embryo have very considerable powers of regu- 

 lation, and two complete embryos can sometimes be produced by a 

 single egg if, in some way or another, the developing system can be broken 

 into two effectively separate parts. A mechanical bending of the egg is in 

 some cases sufficient to spht the yolk into two parts, and if this happens 



^ General reviews: Seidel 1952(1, 1953; Richards and Miller 1937, Krause 1939. 



