646 



ZOOLOGY. 



dents or abnormal products from parents who have been 

 affected by them, the results are usually abnormal, more or 

 less distorted forms, with greater or less defects ; and here 

 again have been observed laws governing the production of 

 abnormalities, the study of these being called teratology. 



We may study the mode of development of the domestic 

 fowl or hen as the best known example to illustrate the 

 changes undergone by an embryo vertebrate, for this pur- 

 pose condensing the statements of Foster and Balfour in 

 their " Elements of Embryology." 



Fig. 540. Blastodermic disk and germ of a rabbit about one day old. seen from the 

 back, a, edge of the head-end of the amnion; b, fore-brain; c, lateral expansion of 

 the same, or primitive eye-vesicle; d, middle, e, hind brain. There are eight protover- 

 tebne, between which is situated the spinal cord. Enlarged ten times. After Bischoff. 



First Day. After fertilization of the egg, segmentation of 

 the egg occurs, but instead of being total, forming a morula 

 or mulberry mass, it is, as in all birds and in the majority of 

 fishes and reptiles (except the lanceletand lamprey eel), par- 

 tial, or confined to the periphery of the yolk, resulting in 

 the formation of a blastoderm, the oval more apparent por- 

 tion being called the " blastodermic disk," which is the be- 

 ginning of the embryo. In six or eight hours after fertili- 

 zation the three germ-layers appear. From the outer germ- 



