GERM-CELLS. 25 



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

 yers 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. 



Germ-cells. 



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

 ing 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 segmen- 

 tation spheres, and divide rapidly and repeatedly. They are termed somatic cells 

 and 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 somewhat 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 five vertebrates, three fishes, the teleosts 

 Cymatogaster and Micrometrus, the elasmobranch Squalus acanthias, and in the 

 frog and turtle. In these five forms the germ-cells arise during segmentation, and 

 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 and which thus becomes the so-called 

 "germinal epithelium." 



The most accurate information we have refers to their development in the dog- 

 fish. In this species the germ-cells are delaminated from the entoderm together 

 with other cells of the mesoderm, and cannot, with our present knowledge, be dis- 

 tinguished from other mesodermic cells. They soon, however, become recognizable, 

 because while the majority of the mesodermic cells are passing into the second 

 stage (compare the section on Mesenchyma, page 89) these germ-cells change but 

 little, if at all, so that they can be recognized as something distinct from the neigh- 

 boring cells. For a short time they are found gathered into tw r o compact groups 

 (Fig. i, Germ-cells] symmetrically placed in the extra-embryonic region, but not far 



