344 1NSECTA. 



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 be- 

 lieves 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, hy). 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 glandular 

 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 Chirononms 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 

 mass of yolk external to the mesenteron takes the form of a median and 

 two lateral streaks. Some of the yolk cells either prior to the establish- 

 ment of the mesenteron, or derived from the unenclosed portions of the 

 yolk, pass into the developing organs (Dohrn, 408) and serve as a kind of 

 nutritive cell. They also form blood corpuscles and connective-tissue 

 elements. Such yolk cells may be compared to the peculiar bodies de- 

 scribed by Reichenbach in Astacus, which form the secondary mesoblast. 

 Similar cells play a very important part in the development of Spiders. 



Generative Organs. The observations on the development of the 

 generative organs are somewhat scanty. In Diptera certain cells known 

 as the pole cells are stated by both Metschuikoff (No. 423) and Leuck- 

 art to give rise to the generative organs. The cells in question (in 

 Chironomus and Musca vomitoria (Weismann, No. 430) appear at the 

 hinder end of the ovum before any other cells of the blastoderm. They 

 soon separate from the blastoderm and increase by division. In the 

 embryo, produced by the viviparous larva of Cecidomyia, there is at first 

 a single pole cell, which eventually divides into four, and the resulting 

 cells become enclosed within the blastoderm. They next divide into two 

 masses, which are stated by Metschnikoff (No. 423) to become surrounded 



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. 



