48 



EVOLUTION AND GENETICS 



which appear show an equally close resemblance. The brain is 

 first a series of three expansions of the neural tube at the anterior 

 end of the body. Limbs first bud out from the body wall as 

 rounded projections, and digits in turn bud from them in those 

 animals which possess such structures (Fig. 27). The mouth and 

 anus form as invaginations of ectoderm, the stomodaeum and 

 proctodaeum, which meet the endoderm and break through to 

 form the continuous tube of the alimentary tract. The nostrils 



Postcardinal veins 

 Vitelline artcrv, 



Precardinal veins 

 Descending aortce 



Umbilical arteries, 



^Aortic arches i and 2 



Body stalkf 1/ \ ^^==^ / \ \ H^"^^ 



Umbilical veins'' \^__^^ \ Sinus venosus 



Vitelline veins 



Fig. 28. — Lateral aspect of the circulatory system in a human embryo of 

 2.6 mm. Diagrammatic. (From Arey's Developmeidal Analomy, after 

 Felix-Prentiss, with the permission of the W. B. Saunders Company.) 



are at first merely depressions of ectoderm which become asso- 

 ciated with the central nervous system as organs of smell, while 

 in the terrestrial forms they also play a part in respiration, and for 

 this purpose join the stomodaeum. In terrestrial forms the 

 trachea and its subdivisions in the lungs are first only evaginations 

 from the primitive gut. By later subdivision of the stomodaeum, 

 they are more definitely associated above the amphibia with the 

 olfactory part of the respiratory system. The swim bladder of a 

 fish corresponds in origin to the lungs. Glands, such as the liver 

 and pancreas, in all vertebrates bud out from the alimentary tract. 

 The Circulatory System. Particularly noteworthy is the cir- 

 culatory system, for when it once develops tubular arteries and 

 veins and a chambered heart, it is similar in all vertebrate em- 

 bryos. A pair of veins enters the body from the yolk sac and a 

 pair from the region of the allantois. These join the heart, whence 



