62 BERTRAM G. SMITH 



V. The internal history of the egg before cleavage. 



A. Ovogenesis 107 



1. The formation of the follicle and the egg membranes 108 



2. The establishment of polarity, and the progress of axial differ- 



entiation 1 13 



3. Resorption of ovocytes; the follicle cells in a phagocytic role. . 126 



4. Organization of the ovocyte shortly before the appearance of 



the germinal vesicle at the surface 128 



B. Maturation. 



1. The germinal vesicle at the surface 129 



2. The dissolution of the germinal vesicle, and the formation of 



the first polar spindle 130 



3. The second polar spindle 134 



4. The organization of the egg immediately before fertilization. . 137 



C. Fertilization. 



1. History of the egg-nucleus 138 



2. History of the sperm-nucleus. 



(a). Penetration of the egg by the spermatozoon 140 



(b). Polyspermy, and the fate of the supernumerary sper- 

 matozoa 145 



3. Union of the germ-nuclei, and the formation of the first polar 



spindle. 146 



4. Changes in the blastodisc 147 



D. Summary 148 



Bibliography 151 



I. INTRODUCTION 



For more than a generation zoologists have eagerly sought 

 for the embryological material of the hellbender, Cryptobranchus 

 allegheniensis Daudin. Until quite recently these efforts have 

 been conspicuously lacking in success. It seems remarkable that 

 the life history of an animal so large, so abundant in localities 

 easy of access, and so important from a phylogenetic point of 

 view, should so long remain shrouded in mystery. But the same 

 difficulty has been encountered in attempts to work out the 

 natural history of several nearlj^ related forms. Eycleshymer 

 ('06) says: 



After years of persistent and patient effort Professor Whitman finally 

 discovered the nests and eggs of Necturiis. Only those who have for 

 years been baffled in their attempts to obtain the embryological material 

 of other North American urodeles, such as Siren, Amphiuma, and Cryp- 

 tobranchus can properly appreciate the enormity of the task. 



