122 PRINCIPLES OF EMBRYOLOGY 



above, the germ-band is becoming transversely segmented, and from each 

 of the anterior segments, outgrowths protrude and grow into the cephaUc 

 and thoracic appendages of the larva. All the main organs of the embryo 

 are then at least indicated, though they still have much histological differ- 

 entiation to carry out before they are fully developed. By the time the 

 main blocking-out of the embryo is complete, the germ-band may still 

 represent only the ventral surface, the dorsal side being occupied by the 

 mass of yolk. At some stage, often quite a late one, the body wall is com- 

 pleted by the outward growth of the two lateral edges of the germ-band, 

 which eventually meet and fuse at the mid-dorsal line. This fmishes the 

 laying down of the embryo proper. 



In some insects, more or less elaborate extra-embryonic structures are 

 also developed. There are often membranes which cover the embryo and 

 presumably help to protect it from outside influences. These are derived 

 from parts of the blastoderm peripheral to the germ-band, by the forma- 

 tion of folds which eventually meet and fuse above the embryo, leaving 

 an outer layer (or serosa) and an inner (or amnion). There is another and 

 most pecuhar phenomenon to be mentioned, which however plays no 

 part in the actual formation of the embryonic organs, namely the so- 

 called 'blastokinesis'. This is the name for a series of movements by which 

 the germ-band may at some stage be dragged from the surface right into 

 the middle of the yolk mass, only to emerge on to the surface again later. 

 The most strongly developed blastokinetic movements occur in species 

 in which the embryo is small compared with the size of the whole egg, 

 but even in forms in which the embryo occupies almost the entire 

 available space, there are often considerable shiftings of the blastoderm 

 as a whole (cf. Fig. 8.2). 



As was pointed out above, there is within the insect kingdom a very 

 large range of variations around the generalised type which has just been 

 described. These variations fall into a series, from an 'indeterminate type' 

 at one end to a 'determinate type' at the other (Seidel 1936). The indeter- 

 minate type includes the eggs of some of the more primitive groups of 

 insects, such as Orthoptera and Odonata ; it is characterised by eggs which 

 are usually rather large and often elongated, provided with only a thin 

 cortex, and developing an embryo which is small in relation to the egg. 

 The name 'indeterminate' is used because very considerable regulation is 

 possible during the early stages of development. In the determinate type, 

 regulation is very shght or altogether absent. The most typical representa- 

 tives of this group are the Lepidoptera and Diptera, whose eggs have a 

 thicker cortex, which is often regionally speciahsed even before fertihsa- 

 tion. The embryo normally occupies the whole, or at least the greater part 



