waite: antennal glands in homarus americanus. 193 



of the period came from the problematical nature of the organ, and to 

 this was later added the interest occasioned by the prominent place 

 given this system of organs in the discussion of the Annelid ancestry 

 of Vertebrates. 



Since no one has held that the endoderm takes part in the formation 

 of the nephridia in Annelids, the organ presents three possibilities as to 

 origin: (1) that it arises entirely from the ectoderm; (2) that it is 

 wholly mesodermic ; or (3) that both ectoderm and mesoderm contri- 

 bute to its formation. The first of these possibilities has not been ad- 

 vocated by any one, although Wilson ('89) * comes very near to such an 

 interpretation. He says (p. 423) that the nephridia "arise in connec- 

 tion with a continuous cell-cord of ectoblastic origin. . . . Each neph- 

 ric cord terminates behind in a pair of teloblasts derived from the 

 ectoblast. The entire nephric cord is formed by the continued divis- 

 ions of these ' nephroblasts,' which agree precisely with the ' neuro- 

 blasts ' in structure, action, and mode of origin." The nephridium 

 (p. 424) is later invested by a sheath of mesoblastic cells which become 

 the peritoneal investment. Wilson believes that the funnel and the 

 investing cells alone arise from the mesoblast, i. e. separately from the 

 rest of the nephridium, and his conclusions are in the main in agree- 

 ment with those of Whitman ('87, p. 161) in the case of Clepsine. 

 Wilson (p. 426) admits that, in view of the observations of Vejdovsky, 

 he cannot " deny the possibility that the glandular part may be differ- 

 entiated from the somatic mesoblast at a very early period, fusing im- 

 mediately with the cells of the nephric cord, which may give rise only 

 to the end vesicle." If this be admitted, his results agree in general 

 with those of Vejdovsky (see p. 194). 



The other extreme, represented by the second possibility, is reached 

 by Bergh ('88). He finds (pp. 226, 227) in Criodrilus a single funnel-cell 

 differentiated at the point of origin of the septum, from which is prolif- 

 erated a cord of cells extending posteriorly. This is the beginning of 

 the segmental organ, which soon becomes covered with a sheath. The 

 external terminal part (Endstiick) is in no way connected with the epi- 

 dermis (p. 229), the segmental organ arising entirely from the " Haut- 

 muskelplatte." The same condition is found by Bergh ('90, p. 501) in 

 Lnmbricus, in the development of the nephridia in which " Trichter-, 

 Schlingen- und Endabschnitte differenziren sich aus einer einheitlichen 

 Anlage heraus, die in den inneren Muskelplatten ohne Betheiligung der 

 Epidermis entsteht." Bergh finds support in Lehmann's ('87, p. 348) 



1 I refer to his later paper only, as it includes all the essentials of the earlier 

 (Wilson, '87) one. 



