TRACHEATA. 395 



The mesoblast arises in connection with the ventral thickening of the 

 blastoderm, but the details of its formation are not known. Metschnikoff 

 describes a longitudinal furrow which appears very early in Strongylosoma, 

 which is perhaps equivalent to the mesoblastic furrows of Insects, and so 

 connected with the formation of the mesoblast. 



The mesoblast is divided up into a series of protovertebra-like bodies 

 the mesoblastic somites the cavities of which become the body cavity and 

 the walls the muscles and probably the heart. They are (Metschnikoff) 

 prolonged into the legs, though the prolongations become subsequently 

 segmented off from the main masses. The splanchnic mesoblast is, 

 according to Metschnikoff, formed independently of the somites, but this 

 point requires further observation. 



The origin of the hypoblast remains uncertain, but it appears probable 

 that it originates, in a large measure at least, from the yolk segments. In 

 the Chilognatha the mesenteron is formed in the interior of the yolk seg- 

 ments, so that those yolk segments which are not employed in the formation 

 of the alimentary canal lie freely in the body cavity. In the relation of 

 the yolk segments to the alimentary canal the Chilopoda present a strong 

 contrast to the Chilognatha, in that the greater part of the yolk lies 

 within their mesenteron. The mesenteron is at first a closed sack, but is 

 eventually placed in communication with the stomodaeum and the procto- 

 daeum. The Malpighian bodies arise as outgrowths from the blind ex- 

 tremity of the latter. 



BIBLIOGRAPHY. 



(397) G.Newport. "On the Organs of Reproduction and Development of the 

 Myriapoda." Philosophical Transactions, 1841. 



(398) E. Metschnikoff. " Embryologie der doppeltfiissigen Myriapoden (Chi- 

 lognatha)." Zeit.f. wiss. Zool., Vol. xxiv. 1874. 



(399) " Embryologisches iiber Geophilus." Zeit. f. wiss. Zool., Vol. xxv. 



1875- 



(400) Anton Stecker. "Die Anlage d. Keimblatter bei den Diplopoden." 

 Archiv f. mik. Anatomic, Bd. XIV. 1877. 



INSECTA 1 . 



The formation of the embryonic layers in Insects has not 

 been followed out in detail in a large number of types ; but, as 



1 The following classification of the Insecta is employed in this chapter. 

 ( ( i ) Collembola. 



I. Aptera. j (a) Thysanura . 



!(i) Orthoptera genuina (Blatta, Locusta, etc.). 

 (2) pseudoneuroptera (Termes, Ephemera, 



Libellula). 



i(i) Hemiptera heteroptera (Cimex, Notonecta, etc.). 

 (2) ,, homoptera (Aphis, Cicada, etc.). 



(3) parasita (Pediculus, etc.). 



