4 INTRODUCTION. 



Beyond the simple fact that the germinal vesicle moves centrifugally, 

 and eventually disappears in the neighborhood of the animal pole, very 

 little is known respecting the course of maturation and impregnation in 

 the eggs of birds and reptiles ; and the pains-taking researches of Van 

 Beneden have thus far given us only a fragmentary history of this period 

 in the mammalian egg. The careful studies of Van Bambeke and Oscar 

 Hertwig brought to light the existence of pronuclei in the amphibian 

 egg, but the fate of the germinal vesicle remained a mystery till the 

 appearance of 0. Schultze's work early in 1887. The investigations of 

 Bambeke, 0. Hertwig, Roux, and Born have given us the chief points in 

 the history of the spermatozoon in these eggs, and it is now quite certain 

 that fecundation is here monospermatic, and not poly spermatic, as some 

 writers have so steadfastly insisted. 



To the solitary observations of Salensky on the development of the 

 sterlet, we are indebted for what we know respecting these phenomena 

 in the ganoid egg. Kastschenko's preliminary paper announces the 

 existence of polar globules in a number of elasmobranch eggs, but this 

 is about the only important scrap of information yet obtained in the 

 history of the germinal vesicle. 



Oellacher's endeavors to trace the history of the germinal vesicle in 

 the egg of the trout were fruitless, and Kupffer's studies on the same 

 object have only yielded results that call for re-examination. Trans- 

 parent pelagic fish-eggs are more favorable objects for the study of these 

 phenomena; but in this field Hoffmann is the only investigator who has 

 claimed to give their connected history. 



The most successful observations hitherto made on the maturation and 

 impregnation of the vertebrate egg are contained in a recent paper by 

 A. A. Boehm. This paper fills the gaps and clears up the discrepancies 

 in the accounts of Miiller, Calberla, Kupffer and Beneke, and Scott, and 

 gives to the egg of Petromyzon an interest scarcely less than that now 

 taken in the egg of Ascaris megalocephala. 



Most of our knowledge of these phenomena has been obtained from 

 the study of a few invertebrate eggs. But here, for obvious reasons, 

 investigation has been more general; and the later contributions remind 

 us that our search must not be limited to the comparatively few eggs 

 which have been found to present very favorable conditions for observa- 

 tion. Even the more refractory objects, such as the eggs of rotatoria, 



