IXTROD UCTOR Y EMBR YOLOG Y. 



INTRODUCTORY EMBRYOLOGY. 



IN order to understand clearly the structure of a vertebrate, 

 it is well to begin with a short account of some of the phe- 

 nomena of development, since a knowledge of the history of 

 the parts will make their relations, one to another, more com- 

 prehensible. The following outline is given in the briefest 

 manner and in the most generalized form, the various modifica- 

 tions which are found in the different vertebrate groups being 

 ignored. 



All vertebrates reproduce by means of eggs. These eggs 

 are specialized cells, produced by the female, which have the 

 capacity, after impregnation, of developing into an animal like 

 that which produced them. The impregnation consists in the 

 union with the egg of a still more specialized reproductive 

 cell, the spermatozoan, produced by the male ; and it is only 

 after this union (called also fertilization) that development is 

 possible. 



The fertilized egg divides (seg- 

 ments) again and again, the result 

 being that the egg is converted into 

 a many-celled embryo. At first the 

 cells of this embryo are arranged 

 in a single layer, surrounding a cen- 

 tral segmentation cavity (Fig. 4). 

 Next, those cells upon one side of 

 the embryo become pushed inside 

 of the others (invaginated) much in 

 the same way that one might push 

 in one side of a hollow rubber ball, 

 the result being partially to oblit- 

 erate the segmentation cavity, and 

 to differentiate the previous single layer into two. This two- 

 layered embryo is known as a gastrula (Fig. 5). 



FIG. 4. Section of an early 

 stage of the egg of Amblystoma 

 showing the smaller cells at one 

 pole, the larger at the other, 

 and at S the segmentation 

 cavity. 



