OF GENERATION. 



sary, this oxygen be imbibed from the above-mentioned orbit of the 

 egg germ, it can only be distributed by means of the vessels in tho 

 circumference of the entire yolk. The second external membrane lying 

 over the amnion (the same, b, 6,) is a transparent, colourless, simple, 

 structureless tunic, which lies next to the egg shell, and clothes this 

 throughout, with the exception of the above-named space containing 

 air. It consequently corresponds with the membrane lying beneath 

 the shell in birds, viz., the chorion, which is here also as deficient in 

 vessels as among the birds. The resemblance to birds is very evident ; 

 a similar space containing air is also observable in birds' eggs, and, the 

 same as here, the embryo imbibes the oxygen, which it requires for 

 respiration, from the air contained in that space. The allantoid is 

 wanting, and consequently the air vessels take their course upon the 

 exterior surface of the amnion, the yolk bag however is contained 

 within the ventral cavity. A canal to correspond with the navel cord is 

 consequently likewise wanting; the entire yolk bag lies within the 

 ventral cavity, and becomes the intestinal canal and stomach, and it is 

 thence perhaps that the stomach of caterpillars is so monstrously large. 

 The larger the embryo becomes the more distinctly do the several 

 organs display themselves. Interiorly Suckow first observed the intes- 

 tinal canal, almost contemporaneously with the external formation, 

 from the simple reason that so soon as the ventral plates had united at 

 the back the yolk bag must necessarily present itself as the internal 

 nutrimental canal. It is evident that the closing of the anus in many 

 larvae stands in close relation to this reception of the entire yolk 

 bag. Suckow also observed, towards the close of the embryo life, con- 

 strictions upon this internal nutrimental canal, which separated the 

 oesophagus and intestine from the stomach ; until then it remained 

 what it was, a longitudinally distended simple bag. Now appear the 

 first traces of air vessels, in the form of tubes, one of which runs on 

 each side of the body, and from division to division sends forth fasciculi 

 of branches, which spread themselves to the intestinal canal. But 

 during the embryo life the tracheae do not enter into action, the stig- 

 mata are consequently closed, and their function commences only upon 

 the exclusion from the egg. The dorsal vessel also developes itself and 

 gradually commences action, at least distinct pulsations have been 

 observed in embryos shortly prior to their quitting the egg shell. The 

 sexual organs are also observed during the last few days of the embryo 

 period, they present themselves in both sexes as small knobs with 



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