IIISTOIJY Of TIM: QBBN I.\N 1:1: 1 HBOBY. 11.') 



First, tin- hedy-sacs arc detached from the I'm da m. nt of tin' 

 cl orda and tin- -ntoblaM . \\ hereupon t he edgefl "f the 

 parietal and \ i>cer.il lamell.-e. thus set five. l'u-e \\ith 



cad) Ut ll !'. 



Secondly, the fundament of tin- chord i i> bent into a chordal 

 groove, and this i^ <-on\ erted into a solid rod, which is 

 completely i-olatt d I'roin tin- entohl.. 



Thirdly, t In- riitoll:i>t do0efl to^rtln-r intt. a tul>.- with a d<>r-;il 



raphe, 



5. The development of the three fundament-, as also that of 

 various other organs, lu-^in> at tin- h ad-end of the inln-yo, and 

 id\ani-es from hei-e toward tin- lila.stojmre, where for a l<mr time u 

 continual formation of new jiai'ts and an increase in the longitudinal 

 growth of the body take place. 



6. During the development of the middle germ-layer, the blasto- 

 pore of the Amphil>ian>. Ki>he>. K.|>;il.>, I'.nd-. and Mammal- ha- 

 ! en metamorphosed into a groove occupying the longitudinal axis 

 of the embryo (primitive groove of the hiu. 



7. The blastopore and the primitive groove in later -t.nres of 

 development undergo degeneration, and are not converted into any 



oig.in of the adult. (For the d.-t.iils of thi-, 966 I'art II.) 



8. Before their disappearance the hla.-topore and primitive groove 



are BlIITOUnded by the medullary folds and taken into the icrn.inal 

 part of the neural tube, whereby a direct communication between 

 neural tube and intestinal tube the neuren teric canal i- effected. 

 The two orv'an-. which communicate with each other fora long time. 

 are later separated by its closure. 



CHAPTER VII. 

 HISTOHY OF i in: &JSBM-LAYBR 



'I'm: fundamental facts of the sheet -like Mructure of the vertebrate 

 body, which have been treated of in the two precedin- chapters, are 

 epitomised as the doctrine of the germ -layers, or the germ-layer 

 tneory. Since this theory is .f the m>t far-reaciiing >igniticanc 

 for the com pi ehenaion of the evolution of form in animals, and can 



in 1 plai-ed side by side with the cell-ti 'jr.|iial with : 



1 devote a separate chapter to its hi>t 



10 



