436 DEVELOPMENT OF ORGANS. 



In Asellns (Ratlike (No. 501), Dohrn (No. 500), Van Beneden (No. 497)) 

 this organ originates as two cellular masses at the sides of the body ju.st 

 behind the region of the procephalic lobes. Each of them becomes tri- 

 foliate and bends towards the ventral surface. In each of their lobes a 

 cavity arises and finally the three cavities unite, forming a trilobed cavity 

 open to the yolk. This organ eventually becomes so large that it 

 breaks through the egg membranes and projects at the sides of the embryo 

 (fig. 243). Though formed before the appendages it does not attain 

 its full development till considerably after the latter have become well 

 established. 



In Mysis it appears during the Nauplius stage as a pair of cavities 

 lined by columnar cells, which atrophy very early. 



Various attempts have been made to identify organs in other Arthropod 

 embryos with the dorsal organ of the Crustacea, but the only organ at all 

 similar which has so far been described is one found in the embryo of 

 Linguatula (vide Chapter xix.), but there is no reason to think that this 

 organ is really homologous with the dorsal organ of the Crustacea. 



The mesoblast, The mesoblast in the types so far investigated 

 arises from the same cells as the hypoblast, and appears as a some- 

 what irregular layer between the epiblast and the hypoblast. It gives 

 rise to the same parts as in other forms, but it is remarkable that it 

 does not, in most Decapods and Isopods (and so far we do not know 

 about other forms), become divided into somites, at any rate with the 

 same distinctness that is usual in Annelids and Arthropods. Not only 

 so, but there is at first no marked division into a somatic and 

 splanchnic layer with an intervening body cavity. Some of the cells 

 become differentiated into the muscles of the body wall and limbs; and 

 other cells, usually ill the form of a very thin layer, into the muscles 

 of the alimentary tract. In the tail of Palcemon Bobretzky noticed 

 that the cells about to form the muscles of the body were imperfectly 

 divided into cubical masses corresponding with the segments; which 

 however, in the absence of a central cavity, differed from typical 

 mesoblastic somites. In Mysis Metschnikoff states that the mesoblast 

 becomes broken up into distinct somites. Further investigations on 

 this subject are required. The body cavity has the form of irregular 

 blood sinuses amongst the internal organs. 



Heart. The origin and development of the heart and vascular system 

 are but very imperfectly known. 



In Phyllopods (Branchipus) Glaus (No. 454) has shewn that the heart 

 is formed by the coalescence of the lateral parts of the mesoblast of the 

 ventral plates. The chambers are formed successively as the segments to 

 which they belong are established, and the anterior chambers are in full 

 activity while the posterior are not yet formed. 



In Astacus and Palsemon, Bobretzky finds that at the stage before the 

 heart definitely appears there may be seen a solid mass of mesoblast cells 

 in the position which it eventually occupies 1 ; and considers it probable 



1 Eeichenbach describes these cells, and states that there is a thickening of the 

 epiblast adjoining them. In one place he states that the heart arises from this thicken- 

 ing of epiblast, and in another that it arises from the mesoblast. An epiblastic origin 

 of the heart is extremely improbable. 



