COMPARATIVE ANATOMY. 



BJ)). 1 Consisting at first t a single layer of cells, the blastoderm 

 lain- on becomes t\vi>- and tlien three -layered. From the relative 

 positions of these, they are spoken of respectively as the outer, 

 middle, and inner germinal layers, or as epiblast, mesoblast, 

 and hypoblast.-' 



The <|iie.-tioii as to the origin of tin- germinal layers, on account of its im- 

 portant signification, is one of the most burning problems in Morphology, and 

 ;ii yet we, cannot arrive at any full and satisfactory conclusion on tin- subject. 

 1 1 may, however, ! allinned with certainty that in all Vertebrates the blast<>- 

 sphere passes or did so in earlier times into a stage called the Gastrula/ 

 One mu.-t imagine this form as being derived primitively from the blastula 

 by .-ui.iH.sing thai the wall of the latter : Fig. 4, Bl>) became pushed in, or 



Sip 



ZX/ 



Fn;. 5. GASTIU i. \. 

 Kid, cpil.lnst ; Ent, hypohlast ; Blp, blastopoiv ; f, ai-ehi-ntemii. 



invaginateil, at uue part, thus giving rise to a double-walled sac ( Fig. ">). The 

 nutiT wall then represents the epiblasl (/,'//), which functions as an organ of 

 ]ii-otectinii and sensation, while the inner, or liypolilast ( l-'.nl . encloses a central 

 space, the primitive inte.-tinal easily (a rcheiit eron), and represent the 

 as.-imilating and digestive ].rimary alimentary canal. The opening . f the 

 to the exterior, where the two germinal layers are continuous, ivpn-.-ents 

 primitive mouth, and is called the blastopore (Fig. 5, ////<). 



Out of the c]iiltlast arise later the epidermis and its derivatives, 



as well as the entire nervous system. The latter is t'ormeil as an 



1 In men. Mastic Vi-rtrlmiti- ova the l.laMoilcnn only cxtciiils part nf tin- \vay 

 tie- pi-iiphcry nt' the nviini IML,'. 3). 



The tiTins ectoderm, mesoderm, .'"id endoderm "' apj.lic.i to th ...... rre- 



^' l.-iy.-r- in an adult animal. 



3 Tip' ]ir..cr,s of the formation df the g.-i<tnil:i may l.e actually observed al tin- 

 i lav in tin' liolnlila .tic ova "I certain Kislie- ace note, ]>. ." and of Amphibia, 

 and the saiiH- process essentially occurs in the meroblastic ova of other Fishes, though 



it is h"iv lie. iv liillicnlt to recognise. In the case "I the Amniofa the difficulty is 

 still greater, but although we ha\c no direct proof nf tli.- existence of a gastmlfl 

 S\ tge, tin' intimat.' coiin.-e! i,,n of (lie developmental process,'- tliroii^lh.iit the animal 

 kingdom render.- it '/ /<//"// certain that the gastrula is represented ill them. 



