DEVELOPMENT OF Till: T\\" PBXMAB1 OBRM LAI 97 



an infolding. Only l>y mean- of tin- tin-.. .-y of infolding can be ex- 

 plain* d tin- dill- ivnt conditions of the anterior and 



ol tin- _T' mi-disc, tip- mop- active tvll-irrowth in '!"' t.-rniory of the 

 ui. i he <xi-t.nc-' of a crescentic groove, ami tin- continuity 

 of the two primary ,'.-nn lay. rs which is .1 -moi-st rable in tliat 

 r.LMon. Only by menus of this theory, finally, is tin- relation of 

 Birds to i he lower cl; ss-js of the Vertebrates ma. I.- j>o>.-iMr. 



The difiCOVeriea which Ki I-ITKK r\i> I:I:M: KI: liave made in their 

 inv. st illations of R"ptili s, which are so closely related to Bird-. 

 contribute to the elucidation of the pending controversy. In the case 

 icerta agilis (fig, 55), Einys europwa. etc., th-r.' i- foiiml. afl in 

 the cas? of the Hen at a corresponding stage of development, at the 

 boundary of the pellucid and opaque areas of the posterior end of 

 th< ^.Tin-disc, an i-xubcrnnt cell-growth in the form of a crescent (a). 

 In the middle plane and slightly in front of this crescent there is 

 to be seen a sttntll, transversely placed, fissure-like </></iing(u), ///,/', ~/ t 



t into a blind sac and is comparable to the crescentic groove. 

 Ki i i i 1:1; i-iii'htly interprets the opening as the blastopore, which is 

 . IK losed between an anterior and a posterior lip, and the cavity as- 

 the cu'h-ntt'roii. He also draws a comparison between the corre- 



iding structures in Birds and Reptiles.* 

 Let us now direct our attention to the succeeding developmental 



s of the germ-disc of the Chick. These con-i-' . chiefly, in. 

 a constant incr. a- of the superficial extent of the disc. 



In tin- freshly laid, unincubated egg (fig. 54) the outer germ-layer 

 (/i) is composed of a single sheet of closely united cylindrical cells ; 

 tin- inner layer (ik), on the contrary, consists of a two-layered to 

 tin-. lay. r. .1 bed of somewhat flattened elements, which are only 



loos, ly associated. 



I'ndi r the influence of incubation the superficial extension of the 



ir nu-disc makes rapid advances (fig. 56). In this process tin outer 



.-layer (ak) OiUMiip- tin- inn- r, and t rininat. s in a ivirion >t the 



* In the interpretation of the manner in which the invauinatiuii t 

 in the case of the eggs of Reptiles and I'.inl-, I differ from other investigators 



also maintain that a irastiulation takes place (< 



HA i" HER, BALFOUlJ, and others). They reirard the whole in ir^in of the germ- 

 cUac as the blastopore, al which the outer trerm-layer bends over to become 

 . untinuous with tin- inner layer. Ai-ennling to my intorpretation. the invagina- 

 tion occurs at a small circumscril)id place of :. T'ne blastopore is 



irom the betrinning surrounded by cells both on its anterior aid it> p isterior lip. 

 The relation of the blastopore as well as that of the geim-layers to the yolk 

 will be more fully dealt with hereafter. 



7 



