22 INTRODUCTION 



EMBRYOLOGY. 



A vertebrate normally begins life as a single cell, the zygote, 

 which is formed by fusion of a sperm cell and an ovum in the process 

 of fertilization. Almost immediately the zygote begins dividing 

 by mitosis to form an increasing number of cells of decreasing size. 

 Each cell so formed in these early divisions is called a blastomere, 

 and the process itself is termed cleavage. The amount of yolk 

 originally stored in the ovum varies in different classes of verte- 

 brates, and since the presence of yolk modifies cleavage and later 

 developmental stages by hindering mitotic activity where it is 

 abundant, the morphology of later stages varies. Nevertheless, 

 homologous stages in development can be recognized. 



Amphioxus.— The relatively simple development of Amphioxus is 

 usually studied as exemplifying clearly the early stages, since there 



Ectoderm 

 Endoderm 

 Archenteron 



— Blastopore 

 8 CELL STAGE 32 CELL STAGE BLASTULA GASTRULA 



Fig. 2. — Diagram of early stages in development of Amphioxus. (Modified from 

 Arey's Developmental Anatomy, W. B. Saunders Company.) 



is only a very small amoimt of yolk stored in the ovum. The yolk 

 present is localized toward one end of the cell, known as the vegetal 

 pole, and at the opposite end, the animal pole, protoplasm is more 

 concentrated. However, the yolk is so small in amount that mitotic 

 activity is little affected and nearly equal blastomeres are formed 

 by cleavage. As mitosis continues, the cells become more numer- 

 ous and smaller, those toward the vegetal pole becoming slightly 

 larger than those at the animal pole. An the cells continue to 

 divide, they maintain a peripheral ])osition and enclose a central 

 cavity. (Fig. 2.) Thus a hollow sphere, the bJastiiJa, is formed, 

 with its wall composed of a single layer of very small cells enclosing 

 a central segmentation cavity filled with liquid. Mitotic acti\'ity 

 continues following blastula formation, and the slightly larger 

 vegetal cells are folded into the segmentation cavity. Invagination 

 of these cells continues to form an inner layer adjoining the outer 

 cell layer and practically obliterating the segmentation ca\'ity, thus 



