GERM-CELLS. 39 



no real explanation to say that it is the result of heredity, for that leaves us as 

 completely in the dark as ever as to the physiological factors at work in the devel- 

 oping individual. 



The conception that the development of an animal depends fundamentally 

 upon th£ unequal expansion and consequent foldings and bendings of the germ- 

 layers was first suggested by the researches of C. F. Wolff on the development of 

 the intestine, and was more clearly recognized by Pander, who definitely asserted 

 that the formation of the embryo is effected by foldings of the germ-layers, and 

 the truth of Pander's view was conclusively demonstrated by C. E. von Baer in 

 1828. In recent times His has studied the problem very intently, and in his 

 memoir on the chick discussed it minutely. In this memoir is to be found most 

 of what little we know of this aspect of embryological mechanics. 



Qerm=cells. 



Recent investigations have made it probable that a few cells are set apart 

 during the period of segmentation to form the germ-cells. Their number is 

 small; they preserve for some time the appearance of segmentation spheres, as 

 the cells which are formed during the segmentation of the ovum are sometimes 

 called. They multiply very slowly during the earliest stages of development. 

 A great majority of the cells produced during segmentation lose the character of 

 segmentation spheres, and divide rapidly and repeatedly. The cells belonging 

 to the class of this majority form the various tissues of the body. The germ- 

 cells, on the contrary, seem to multiply very slowly and never to become very 

 numerous in the embryo. As they multiply they separate from one another and 

 become more or less completely surrounded by tissue cells. They pursue their 

 development, one is tempted to say, independently of tissue formation and some- 

 what like foreign members of the body. We put, accordingly, the germ-cells in 

 a class by themselves in contrast to the body or somatic cells. 



Our actual knowledge of the history of the germ-cells is very incomplete. 

 The statements just made about them are based on observations on very few 

 animals. Their exact origin has been traced only in three species of vertebrates, 

 all fishes, the teleosts Cymatogaster and Micrometrus, and the elasmobranch 

 Squalus acanthias. In these three forms the germ-cells arise during segmenta- 

 tion, remain more or less closely together, or segregated, during the earliest 

 stages. They then separate from one another and gradually migrate into the 

 epithelium, which covers the anlage of the genital gland, which thus becomes 

 the so-called "germinal epithelium." 



The existence of the germinal epithelium has long been known, and its 

 characteristics have been described in all recent text-books of embryology. The 



