vol. 3.] Robertson. — Embryonic Fission in Orisia. 121 



"brown bodies" of the sexes is not so apparent. In both the 

 residue becomes pushed into the extreme base of the zooeeium 



and is packed into smaller space. 



The evidence for degeneration which is afforded by the 

 scarcity of spermatozoa, and by the resemblance between the 

 "brown body" of the male colonies and the testis, is strength- 

 ened by its probable correlation with what occurs in the 

 female colonies. Here, as will be shown, large numbers 

 of ova are produced, but on account of the reproduction 

 peculiar to Orisia, relatively few give rise to larva', hence a 

 relatively small number of sperm are functionally necessary. 

 Degeneration of the male genital product, if it occur, is to be 

 regarded, then, as a secondary modification correlated with the 

 fact that every egg that contributes to the perpetuation of the 

 species produces, through embryonic fission, not one, but a great 

 many colonies. 



2. Origin of the Femah Genital Products. — In the female 

 colonies of Orisia eiurnea the ova arise as do the male germ cells, 

 from the mesoderm of the growing tip of the branches. 

 They are differentiated at the tip of the branches, and in 

 no other part <>/ the colony. PI. XII, Fig. 8, represents a 

 section from the ventral side of a female colony, in which 

 the two layers of the body wall are distinctly shown. Close 

 to the anterior edge is a row of small round ectodermal cells 

 (ec.cls.), forming the outer layer, while inside of this is a 

 layer of larger cells possessing very large distinct nuclei, and 

 constituting the mesodermal layer (mes. els.). The cells of this 

 layer perform various roles in the economy of the colony, 

 some giving rise to part of the parietal lining of the zooeeia, 

 some being transformed into the mesenchymatous tissue of the 

 branches (mes. fix.), and the remainder produciug the germinal 

 epithelium. If a comparison lie made between Fig. 8 of a female 

 colony, and Fig. 2 and 3 of a male colony, no difference will be 

 recognized in the cells of this tissue. In both, the germinal cells 

 are of the same size, and bear identical relations to the growing 

 points. It was shown for the male colonies that the germinal cells 

 are more numerous in the angles, right and left, of the tip. This 

 is true also of the female colonies, as may be seen in PI. XII, 



