01 Vm tWO MMM.I.I- QKBM LAYERS. 1-1 



lir>t the changes which may he 1 in vi.-\\inu* the '-"-mi li-<- 



from tin- surface. .-iii.l to thesv >hall :nll, secondly. the moro im 

 j)i-t:tnt n.-ult- aci|iiircd by >eries of cross sections, 



At the po-terior margin f tin- L" rm di-c of tin- Chirk (tig. Si A), 

 which con-ist- .it' two layer- lying mi the yolk like a watch-glass, we 

 had distinguished not only a short time be fort- incubation, but al-o 

 during t he early hours of that process the crescent (*) and tin- 

 ci-ocfiit ir irroovr. an<l had learnr.l to n-coirnix 1 that this \\a- tin- 

 place from wliich the imiiT p'l'lii lay.-r ATOBe lv a j.roc.-> of 1'oldin.ir 



under. 



When, during the first hours of iiu-nhat ion. tle irerni layer- <^row 

 out farther on the yolk, the crescentic groove (fig. 81 B) is con- 

 vt-ned into the primitive groove (pr). a -nurture ( ,f far-reachini: 

 significance. 



Tin- iiietaiuor].h.i>. according to the ezoellei lieeof \^ 



take> place in the following manner : In the middle of the anterior 

 blast oporic lip, where the outer germ-layer bends over to become 

 continuous with the inner, there ari-e> a small notch, which i- 

 directed forwards (fig. 81 A sk) ; this gradually eloniratfs into .-, 

 groove (fig. 81 /?), corresponding with the future longitudinal axi> of 

 theenihryo. and hy the following method : the right and the left halve- 

 of the [anterior] )>la>toporic lip. to^-ther with the part which bounds 

 the lirM notch, grow toward each other, and conic in contact with 

 <-uch other in the median plane, with the same lapidity with which the 

 disc inciva-e> in super- 



ficial extent. Foratinie, ,----. 



therefore, the blastopore . -/-":; . 



l:a- the form of a short />'""V\ 

 Inn git inlinal groove, { 

 which, at its posterior ^>*^^' 



end, is bent around into F ig. 82. Diagrams to elucidate the formation of theprimi- 

 two >hort tian.-vrrselv tive groove. ;ift,r IM-V.M, 



Tli.- iiicr.Msiii- si/..- ..f rli.! |nn-diaQ in the course of the 

 plaoed crt'-c.-ntic horns dcrtlopnnt to iadintadtf dotted droaUrBaai. The 



(s] Kinallv t he>e aUo heuv - v 'i'"- ' T 



primitive gio,.ve which arisos fr.nu it by the fusion of 



have disappeared ; they, the edges of the crescent 



too, ha\e grown toward 



each other, toward the median plan.-, and have thus contributed 



largely to the posterior elongation of the primitive groove. By thi.s 



remarkable process of irrowth the whole blastopore i- converted from 



a transverse Insure inio a Longitudinal one. 



The accompanying diau'i n rve to illustrate this highly 



