ZOOLOOY AND BOTANY, MICROSCOPY, ETC. 39 



the beginning of the third day they are sharply segregated into two 

 bilateral cords situated in the endoderm of the area pellucida, laterally, 

 in the caudal half of the disc. In the two-day embryo they extend 

 from the neurenteric canal to the end of the primitive streak ; in the 

 three-day embryo from the sixth somite to the caudal extremity of the 

 streak. The cords become more lineally placed, make a linear 

 connexion with the overlying visceral mesoderm, and their cells 

 migrate during the fifth day into this mesoderm and thence medially 

 (during the sixth and seventh days) towards the root of the forming 

 mesentery of the closing hind-gut. Individual cells migrate medially 

 also within, or back into, the endoderm of the gut. The germ-cells in 

 the medial endoderm become included in the mucosa of the closed hind- 

 gut, those in the mesoderm in the enveloping raesenchyma and the gut- 

 end of the mesentery. From these locations the majority of the germ- 

 cells subsequently (seventh to twelfth day) migrate up the mesentery 

 and across the coelomic angle into the future sex-gland. They become 

 incorporated among the mesenchymal cells of the gland and the covering 

 peritoneal epithelium, where they suffer no striking change in form, 

 size or content, at least up to the thirty-second day of incubation. The 

 germ-cells migrate by amoeboid activity, probably assisted in small part 

 by the factor of unequal growth. The migration period is not sharply 

 limited, but practically ceases by the sixteenth day. A certain number 

 of migrating germ-cells go astray, and most of these probably degenerate 

 in situ, but some may persist to form, under appropriate stimulus, a 

 focus of neoplastic growth. The total number of germ-cells counted in 

 a twelve-day embryo was 352, the number within the gonads being 

 about equally divided (118 left, 127 right). Occasional cells may 

 divide by mitosis or undergo degeneration at any stage. No germ-cells 

 were found contributing to the formation of the Wolffian duct. The 

 germ-cells do not differ from young somatic cells in character of mito- 

 chondrial content. No transition stages between coelomic epithelial 

 cells and germ-cells appear up to the thirty-second day, and no secure 

 histological basis can here be found for separating the germ-cells of the 

 gonads into large " primary genital cells " and smaller " secondary 

 genital cells " (Felix) or " gonocytes " (Dustin). 



The evidence derived from a study of the Garetta embryos is in 

 complete harmony with the idea of a single uninterrupted line of sex- 

 cells from primordial germ-cells to oogonia and spermatogonia, and with 

 the hypothesis of a vertebrate Keimbahn or continuous germinal path. 

 The fact of fundamental significance with respect to the primordial 

 germ-cells is their original extra-regional distribution, and their genetic 

 independence of the soma-cells, 



b. Histology. 



Differentiation of Cells in the Developing Organism.* — Vera 

 Dancharoff discusses the problem whether the relations of differently 

 organized tissue which work together in symbiosis are definitely deter- 

 mined by their constitutional specificity, or whether there exist in the 



* Amer. Nat., li. (1917) pp. 419-2S. 



