382 0, 0. WHITMAN. 



horns' incubation, in the posterior half of the area pellucida ; and 

 both agree that this area is continuous at its base with the area 

 opaca, or germinal ring. According to Rauber, the cells consti- 

 tuting the lunula, by a centripetal movement, arrange them- 

 selves along each side of the longitudinal axis of the future 

 embryo, and thus give rise to a " bilateral string," the so-called 

 primitive streak. The streak represents simply the united 

 edges of a small portion of the blastoporic rim ("properistome," 

 Haeckel), and this mode of origin of the embryo is called " sto- 

 matogenic." The streak, groove, and marginal notch are re- 

 garded as so many " phenomena of conjunction." The area 

 opaca is held by Rauber to be homologous with the germinal 

 ring of the Fishes, and here, as in the Elasmobranch, a posterior 

 ''embryoplastic" portion (about one third of the entire ring) 

 is distinguished from the anterior non-embryoplastic {" peri- 

 embryonal'^) portion (No. 34). 



His distinguishes two concentric rings in the area opaca, or 

 " ring-area." " The ring-area of the embryonic disc accord- 

 ingly now consists of an inner broad and opaque germinal-wall 

 portion and a thin transparent margin (or secondary marginal 

 rim) ; both are distinguishable with the naked eye " (No. 22, 

 p. 165). These two zones of the area opaca are plainly seen 

 in the blastoderm I have described. The inner thicker zone is 

 the one which contains the embryoplastic material. 



From the above citations it is evident that His and Rauber 

 are in perfect accord on the main question ; and, while both 

 claim that the vertebrate embryo arises by the concres- 

 cence of the homotypical halves of the germinal ring, 

 neither has anywhere intimated that intussusceptional growth 

 could not go on at the same time. On the contrary, Rauber 

 has expressly stated it as a self-evident fact that such growth 

 is a concomitant of the process of concrescence (No. 39, 

 p. 66). 



In opposition to the view taken by His and Rauber we 

 have the differentiation theory put forward by Balfour. In 

 the second volume of his ' Treatise on Comparative Em- 

 bryology' Balfour has summed up his latest conclusions on this 



