602 MONTGOMERY— MORPHOLOGY OF THE [April 24, 



probable correctness of this distinction is amply substantiated by 

 the cell-lineagists, and the arguments for it have been well presented 

 by Torrey. The mesectoblast is of ectoblastic origin, it is in part 

 equivalent to the mesenchyme of the Hertwigs ; it forms larval and 

 to less extent adult structures, but never gives rise to germ cells. 

 The mesentoblast form adult structures and contains the germ cells. 

 These again are fundamental differences, so that it is no longer 

 sufificient to state a part is mesoblastic, it is necessary to know 

 whether it is mesectoblastic or mesentoblastic. The mesectoblast is 

 in reality an emigrant or delaminant of the ectoblast, it is genetically 

 related with that layer and not with the mesoblast. 



Relation of position to, and origin from, these four embryonic 

 layers gives then a primary criterion for deciding the homologies of 

 the excretory organs. And these relations of position involve also 

 place-relations with regard to the primary cavities of the body: The 

 blastocoel, the space between ectoblast and mesoblast ; the ccelom, the 

 space lined bv mesentoblast ; and the gastroccel, the space lined by 

 entoblast. 



Using the relations of position and origin as of primary im- 

 portance, and anatomical and histological relations as of secondary, 

 w^e will proceed to arrange the excretory organs in genetic groups. 

 Many of the organs described in the preceding part of this paper 

 could not be entered here on account of the insufficiency of our 

 knowledge concerning them ; and some others have to be marked 

 doubtful for the same reason. It is at the best a hazardous under- 

 taking to classify other men's results, and the danger is multiplied 

 when descriptions are imperfect. ^- 



(a) Wholly Ectoblastic Excretory Organs, not Opening into the 

 Ca^loni and not Serving as Genital Ducts. 



I. Hypodermal skin glands. These are perhaps the most 

 primitive excretory organs, and are of wide distriljution. Excre- 

 tory function of them has been proved for Pycnogonids, Insects, 

 Arachnids, Vertebrates and certain others; but probably most hypo- 

 dermal glands are rather secretory than excretory. 



" Here may be mentioned a generalized embryonic excretory organ, the 

 blastocoel, which Kofoid has shown to have the value of a discharging vesicle 

 and to continue that function up to the gastrula stage; Meisenheimer has ac- 

 cepted Kofoid's conclusions. 



