M.\ i LOPMBV i "i nn: TWO KIDD 1 1 ; > 



inner ireriu la\er (ectoblast, entol.l.-tM ) ; tin- ti tin- M 



two layers i> the ol.litel.'it. d rlea \ :. L''-ra \ ilV ; tin- ea \ it V nsnltil^ 



from tlif invagination N tin- cirK-iitfroii.it- external opening tin- 

 primitive mouth (blastopore, prostoma, crescentic groove, primitive 



3. The four kinds of gastrulae correspond to tin four 

 Mast 



(a) In Amphioxus the coelenteron is wide, and each germ- 



layer i> made up of a singi- f cylindiical celN. 



(b) In Cyclostomes and Amphibia the mass of yolk i 



accumulated on the vrntral nail of the cu-lmt-Tmi in 

 the inner germ layer, and causes a protuberance, by 

 means of which the ca-lnittTon is reduced to a fi^- 



(c) In Fishes, Reptiles, and Birds the process of invap:, 



n mains confined to the germ-disc, since the unsegmentr.l 

 yolk, on account of its cun.-idnable volume, cannot be 

 made to share in the imagination. The cjerm- 

 becomes two-layered by means of an ingrowth of cells 

 at the crescentic groove (blastopore). The yolk at-ijuii B 

 a cellular boundary very slowly and at a late period ; 

 it is overgrown by the margin of the germ '. 

 when the supplementary cleavage (yolk-nuclei) tak. > 

 place* 



The outer germ-layer spreads itself out ai.d -n\vl>p> 

 the yolk most rapidly; then follows the inner, and finally 

 the middle layer. 



(cZ) In Mammals the inner germ-layer is developed from the 

 thickened region of the blastula, probably by means of 

 an invagination, because at a later- stage an orifice of 

 invagination, comparable with the primitive groove of 

 Birds, or a blastopore, can be demonstrated. At the 

 beginning of its development the inner germ-la\ei 

 terminates below in a free margin, so that the ccelen- 

 teron is for a time closed in on the ventral side by the 

 outer germ-layer only, a peculiarity which is comparable 

 with the conditions in Reptiles and 1'irds if we cone 

 the yolk-material to have disappeand in this instance 

 before it is completely surround, d by tin* inner germ- 

 lay 



4. In Vertebrates the gastrula presents a sharply expressed 

 trilateral symmetry, so that one c;m ea-il distinguish the future 



