TRACHEATA. 415 



and further states that these two masses meet first ventrally and much later 

 on the dorsal side. In Lepidoptera, on the other hand, Hatschek finds that 

 the hypoblast arises as a median mass of polygonal cells in the anterior part 

 of the ventral plate. These cells increase by absorbing material from the 

 yolk, and then gradually extend themselves and grow round the yolk. 



Dohrn (No. 408) believes that the yolk cells, the origin of which has 

 already been spoken of, give rise to the hypoblastic walls of the mesenteron, 

 and this view appears to be shared by Graber (No. 412), though the latter 

 author holds that some of the yolk cells are derived by budding from the 

 blastoderm 1 . 



From the analogy of Spiders I am inclined to accept Dohrn's and 

 Graber's view. It appears to me probable that Kowalevsky's observations 

 are to be explained by supposing that the hypoblast plates which he believes 

 to be split off from the mesoblast are really separated from the yolk. 



It will be convenient to add here a few details to what has already been 

 stated as to the origin of the yolk cells. As mentioned above, the central 

 yolk breaks up at a period, which is not constant in the different forms, into 

 polygonal or rounded masses, in each of which a nucleus has in many 

 instances been clearly demonstrated although in others such nuclei have not 

 been made out. It is probable however that nuclei are in all cases really 

 present, and that these masses must be therefore regarded as cells. They 

 constitute in fact the yolk cells. The periphery of the yolk breaks up into 

 cells while the centre is still quite homogeneous. 



The hypoblastic walls of the mesenteron appear to be formed 

 in the first instance laterally (fig. 189 B and C, /if). They then 

 meet ventrally (fig. 185 A and B), and finally close in the 

 mesenteron on the dorsal side. 



The mesenteron is at first a closed sack, independent of both 

 stomodaeum and proctodaeum ; and in the case of the Bee it so 

 remains even after the close of embryonic life. The only gland- 

 ular organs of the mesenteron are the not unfrequent pyloric 

 tubes, which are simple outgrowths of its anterior end. It is 

 possible that in some instances they may be formed in situ 

 around the lateral parts of the yolk. 



In many instances the whole of the yolk is enclosed in the walls of the 

 mesenteron, but in other cases, as in Chironomus and Simulia (Weismann, 

 No. 430 ; Metschnikoff, No. 423), part of the yolk may be left between the 

 ventral wall of the mesenteron and the ventral plate. In Chironomus the 



1 Graber's view on this point may probably be explained by supposing that he has 

 mistaken a passage of yolk cells into the blastoderm for a passage of blastoderm cells 

 into the yolk. The former occurrence takes place, as I have found, largely in Spiders, 

 and probably therefore also occurs in Insects. 



