FORMATION OF THE ORGANS. 159 



accumulation of mesoderm-cells is discerned, forming the first rudi- 

 ment of the heart (Figs. 63, 64, h, pp. 131, 132). 



In early stages the thoracoabdominal rudiment is surrounded by a 

 -clear area bounded externally by closely contiguous blastoderm-cells. 

 The latter soon become raised so as to form a fold which is specially 

 distinct in its lateral parts ; this is the first rudiment of the thoracic 

 shield (Fig. 80, ts). The clear area thus referred to is the rudiment 

 of the branchial cavity. 



A remarkable structure lying like a dorsal disc on the embryo and called the 

 "carapax" has been described in the early stages of Atyephyra by Ischikawa. 



The development of the Decapoda is principally characterised by 

 the fact that the whole mass of food-yolk is confined to the anterior 

 part of the dorsal side of the body, while the thoraco- abdominal 

 rudiment is free from yolk. Even in late stages, in which the form 

 of the hatching animal is already quite distinct, the cephalo-thoracic 

 portion appears swollen up by food-yolk until almost spherical. 



Formation of the Organs. 



Our knowledge of the development of the different organs in the 

 Crustacean embryo is still somewhat limited. The Decapoda are in 

 this respect best known to us from the researches of Bobretzky 

 (Xo. 41), Reichenbach (Nos. 64, 65), and Kingsley (No. 52-55). 

 Besides these we have the observations of Nusbaum (Xo. 39) on 

 Mt/sis, of Bobretzky (No. 80) and Xusbaum (Xo. 85) on Oniscus, 

 of Grobben (Xos. 11 and 21) on Moina and Cetochilus, of Claus 

 (Xos. 8 and 9) on Branchipus, Apus, and others. 



A. External Integument. 



As the superficial ectoderm of the embryo yields on its external 

 surface the chitinous skeleton of the larva, it gradually acquires the 

 character of the hypodermis or the matrical layer of this skeleton. 

 It has been recently proved by T. Tullberg that, in the Lobster, the 

 origin of this chitinous skeleton may be traced back to a direct 

 transformation (chitinisation) of the body of the cell. It is an 

 interesting fact that the hypodermal cells not only are able to change 

 into chitin at their external ends, which are directed to the body 

 surface, but that occasionally even their basal portions are similarly 

 modified. Thus Reichenbach observed that, in Astacus, single 

 hypodermal cells lengthen inwards and grow out into chitinised 

 strands and pillars which function partly as supports for the carapace, 

 and partly as points of attachment for the groups of muscles. In 



