90 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



that the appendages appear in succession from in front backward, and 

 that they are well developed long before the segmentation of the germ 

 baud. The blastoderm is interrupted only by the " dorsal organ," 

 which is attached to the inner egg membrane. 



Claypole ('98, pp. 255-258) distinguishes live egg membranes in 

 Anurida, and maintains that all arise from the egg or the blastoderm. 

 I find that in the ripe egg two are evident : a thick outer and a thin 

 corrugated inner one, respectively analogous to, if not homologous with, 

 the chorion and the vitelline membrane of other insects. Another deli- 

 cate membrane completely envelops the embryo in early stages (Plate 1, 

 Figures 1, 3, mb.), except where interrupted by the dorsal organ. I have 

 found it to be, not a " larval skin," but a blastodermic membrane. 



The peculiar cleavage of Collembola has been observed by Oulganine 

 ('75, '76), Lemoine ('83), Claypole ('98), and Uzel (*98). In the most 

 nearly related group, Thysanura, the cleavage has been shown to be 

 superficial by Grassi ('85), Heymons ('96, '97"), and Uzel ('97, '98). 

 In cleavage, then, Collembola resemble many Crustacea and Arachnida, 

 in which it is at first total and secondarily superficial. 



Thysanura, on the other hand, approach the Orthoptera, in that the 

 cleavage is from the first superficial. 



The ''dorsal" or " precephalic " organ of Collembola has been de- 

 scribed by Lemoine ('82), Wheeler ('93), Claypole ('98), and Uzel ('97, 

 '98) ; of Thysanura, by Grassi ('85), Heymons (*96, '97*), and Uzel 

 ('97, '98). Wheeler homologized it with the " indusium " of Orthoptera, 

 and suggested its analogy with the embryonic sucking-disk of Clepsine. 

 Claypole collected evidence of a similar structure in Crustacea, which 

 has been reinforced by Uzel. 



Reference Stages. 



For descriptive purposes I have selected nine consecutive stages of 

 development, which may be identified in the entire egg by the following 

 characteristics : — 



At Stage 1 (Plate 1, Figure 1) the embryo is almost spherical with 

 all the primary appendages represented by small papilla?. The dorsal 

 organ is large, with a spherical imbedded portion and an expanded super- 

 ficial part, the latter firmly attached to the corrugated membrane. This 

 stage is very nearly that of Claypole's ('98, Plate XXIII.) Figures 40 

 and 41 of the same species. 



Stage 2 (Plate 1, Figure 2) is characterized by folds representing the 



