524 



DEVELOPMENT OF ORGANS. 



period. A perhaps homologous structure is found in the embryos of Euphau- 

 sia, Cuma, etc. 



In many Branchiopoda a dorsal organ is found. Its development has 

 been studied by Grob- 

 ben in Moina. It 

 persists in the adult 

 in Branchipus, Lim- 

 nadia, Estherea, etc. 



In the Copepoda 

 a dorsal organ is 

 sometimes found in 

 the embryo ; Grob- 

 ben at any rate be- 

 lieves that he has 

 detected an organ of 

 this nature in the 

 embryo of Cyclops 

 serrulatus. 



A paired organ 

 which appears to be 

 of the same nature 

 has been found in 

 Asellus and Mysis. 



In Asellus (Rathke (No. 501), Dohrn (No. 500), Van Beneden (No. 497)) 

 this organ originates as two cellular masses at the sides of the body just 

 behind the region of the procephalic lobes. Each of them becomes trifoliate 

 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 Lingu- 

 atula (vide Chapter XIX.), but there is no reason to think that this organ is 

 really homologous with the dorsal organ of the Crustacea. 



FIG. 243. DIAGRAMMATIC SECTION OK AN EMBRYO 

 OF ASELLUS AQUATICUS TO SHEW THE PAIRED DORSAL 

 ORGAN. (From Bullar; after E. van Beneden.) 



The mesoblast. The mesoblast in the types so far investi- 

 gated arises from the same cells as the hypoblast, and appears 

 as a somewhat 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 



