vi HEXAPODA EMBRYONIC DEVELOPMENT 493 



exhausted they pass into a pupa-like stage, out of which, however, not the imago, 

 but a new larva emerges. Still further pupa-like stages of development then follow, 

 till at last the final real pupa stage occurs. Here we can very clearly recognise, 

 especially in the first two larval stages, the influence of various modes of life on the 

 larvae of one and the same animal. 



Many Pteromalidce (Hymenoptera) pass through a series of peculiarly shaped larval 

 stages, which are as yet by no means explained. The larvse live parasitically in the 

 eggs, larvse, and pupse of other insects, in which the Pteromalidce lay their eggs by 

 means of an ovipositor. It is remarkable that the youngest larvse possess far less 

 highly developed inner organs than are usually found in the larvse of other insects. 



The above is naturally but a very incomplete description of this most interesting 

 subject. 



B. The Embryonic Development of Insects. 



Hydrophilus, the water beetle, affords us a good illustration of this development. 



The egg is a long oval, with pointed anterior and blunt posterior pole. The 

 segmentation is that typical of the centrolecithal eggs, and leads to the formation of 

 a blastosphere. In this blastosphere we can distinguish a single superficial layer of 

 small cells, the blastoderm, and, enveloped by it, the nutritive yolk with scattered 

 nuclei. 



The formation of the embryo proceeds from one side of the blastosphere only, i. e. 

 from the future ventral side, on which the blastoderm cells are higher than elsewhere. 

 We may call this portion of the blastoderm the embryonic rudiment. At an early 

 stage we can distinguish the boundaries of the segments, appearing externally as 

 transverse streaks or lines. Anteriorly and posteriorly two longitudinal furrows 

 appear, grow towards each other and unite, so as to mark off on the embryonic 

 rudiment a peripheral portion, the lateral plates, from a central portion, the middle 

 plate. The middle plate sinks below the surface, and so forms the floor of a channel- 

 like medio-ventral invagination, whose edges grow towards each other on each side. 

 This invagination is represented in transverse section in Fig. 350, A- Its edge is to 

 be considered as the edge of the blastopore. How the lateral edges of this blastopore 

 approach each other in the middle line and finally fuse with each other is illustrated 

 by Fig. 352, A, B, C. After the closing of the blastopore the invagination becomes 

 a medio-ventral longitudinal tube, over which the blastoderm of the former lateral 

 plates spreads. From this invaginated tube proceed the mesoderm and perhaps also 

 the endoderm (epithelium of the mid-gut). 



Even before the closing of the blastopore the rudiments of the embryonic en- 

 velopes so common among insects, viz. the amnion and the serous envelope, appear. 

 They first appear as a fold of the blastoderm at the edge of the embryonic rudiment. 

 This fold grows more and more from all sides over the embryonic rudiment, 

 finally covering it. The embryonic rudiment thus comes to lie in the base of a cavity 

 whose mouth, wide open at first, grows smaller and smaller by the growing and final 

 closing together of the amnion folds ; the final closing takes place over the anterior 

 end of the embryonic rudiment. The tranverse section B, Fig. 350, shows the rising 

 amnion folds, the transverse section C shows them grown over the embryonic 

 rudiment so as to form a continuous cover. In the surface views of Fig. 352 the 

 folds are denoted by af and of". 



The cavity which is formed by the amnion is called the amnion cavity. Its roof, 

 in correspondence with its origin, consists of 2 epithelial lamellae, an inner one, which 

 at the edge of the embryonic rudiment is continued into its blastoderm and represents 

 the actual amnion, and an outer one, which at the edge of the embryonic rudiment 

 is continued into the blastoderm of the whole remaining surface of the egg, and 



