340 PHYLUM ARTHROPODA. 



segmentation may go on superficially from the beginning, 

 leaving the yolk mass undivided. 



In some cases (Mysis, Scorpio) the blastodermic layer is con- 

 fined after its formation to one pole of the egg the telolecithal 

 eggs of Balfour ; but in Insects and many Crustacea it entirely 

 surrounds the yolk an arrangement which hardly occurs out- 

 side the Arthropoda. For such eggs Balfour adopted the name 

 centroleciihal. 



The remarkable phenomenon known as polyembryony, pre- 

 sented by some parasitic Hymenoptera, in which the egg 

 divides up and gives rise directly to a brood (12-1,000) of 

 new individuals, is referred to on p. 641. 



Associated with the abundance of the yolk in the interior of 

 the egg is the early disappearance of the mesoblastic somites, 

 and the consequent difficulty in tracing the development of 

 the organs derived from the coelom. 



In Peripatus, the Myviapods and apterygote Insects the 

 endoderm furnishes the greater part of the alimentary canal, 

 as in most other animals ; but in some of the higher Arthropods 

 the endodermal portion, the mid-gut proper, is much reduced 

 in length, the stomodaeal and proctodaeal portions (fore- and 

 hind-guts) being proportionally elongated. Thus in some 

 Decapods and Isopods only the hepatic diverticula and a small 

 part of the adjoining central tract of the alimentary canal are 

 endodermal. In Insects the tract extending from the crop to 

 the beginning of the ileum (including the hepatic diverticula but 

 not the malpighian tubes) is usually named mid-gut, the chitin- 

 lined regions in front and behind being formed by the ectodermal 

 involutions. The embryological evidence on this point is however 

 conflicting, and Heymons is even inclined to the view (though 

 this meets with opposition in competent quarters) that the 

 endoderm, though taking part in the formation of the alimentary 

 canal of some of the lower Insects (Lcpisma) is entirely unre- 

 presented in the adults of the higher groups, and that the whole 

 alimentary canal is formed in them by the meeting of stomo- 

 daeum and proctodaeum. 



In connexion with the development may be mentioned the remarkable 

 property of the arthropod body, which it possesses, in common with other 

 groups of the Metazoa, of reproducing parts lost by injury or amputa- 

 tion. Only the limbs and eye-stalks are thus reproduced, and in the case 

 of the thoracic appendages of the Crustacea only the parts distal to the 



