No. 2.] THE EMBRYOLOGY OF THE APTERYGOTA. 75 



cleavage ; it is markedly significant that nothing of this kind 

 occurs in Anurida after the holoblastic condition is lost, thouo-h 

 the yolk is fused into a solid mass and nuclei are scattered 

 through it ; such secondary cleavage is reported in Anurophorus, 

 where ^g^ cleavage is superficial. 



It is equally clear that the amnion and serosa are absent in 

 the Collembola, the embryonic membranes formed having the 

 nature of " Blastodermhauten." In Anurophorus, Achorutes, 

 Degeeria, Sminthurus, and Anurida these membranes show 

 some amount of crenation and hence have powers of expansion. 

 In all cases they are found in connection with the so-called 

 "dorsal organ," which is a structure clearly homologous 

 throughout the Collembola, being similarly placed and similar 

 in development. A similar structure, similarly placed, is also 

 found in Campodea and Japyx. Heymons considers the 

 "dorsal organ," caused by the invaginating cellula envelopes, 

 homologous with these. But the distinct and early appearance 

 of this organ and the simultaneous presence of the amnion and 

 serosa in Japyx and Campodea are clear evidences against such 

 an homology. This is still further confirmed by reference to 

 the structure described by Wheeler as the " indusium "; this is 

 without doubt, as he states, the homologue of the apterygote 

 "dorsal organ," and is certainly distinct from the structure that 

 rises later during the elimination of embryonic envelopes. It 

 is possible that a structure similar to the earlier stages of the 

 indusium may exist in Lepisma, but no such specialized later 

 developments would be expected as those found in the 

 Orthoptera. 



It is also interesting to see the clearness with which certain 

 facts are indicated as to the appearance and fate of certain 

 appendages. The collophore is without doubt a fused pair of 

 abdominal feet ; the spring has a similar origin on the fourth 

 or fifth abdominal segment, and according to Uzel the styli 

 and ventral bladders rise directly from abdominal appendages. 

 There is almost unanimous evidence that an intercalary seg- 

 ment exists in the apterygote head placed between the antennae 

 and the mandibles, disappearing in some cases but remaining to 

 form permanent mouth-parts in others. It is reasonable to 



