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 ¢eratology. 
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 ‘“‘ Hlements 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, 0, fore-brain; ¢, lateral expansion of 
the same, or primitive eye-vesicle; d, middle, e, hind brain. There are eight protover- 
tebre, between which is situated the spinal cord. Enlarged ten times.—After Bischoff. 
First Day.—Afiter 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 lancelet and 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- 
