: .<>r.Mi.vr Of NIK TWO i'i:i.M.\i:\ I.KI:.M I.A , 





(i 



Fig. 44. Gastrula of Amphioxus lanceolatus, after 



II MM HI:K. 

 ak, Outer germ-layer; it, inner germ-lay 



Ha-topore, or mouth of archenteron 



( 17''. tl.r c 1U (/: i an somewhat 1 



the yolk granules lodu r < d iii them. Th. of the forma! i 



tin- irastnila commences at tliis place. The \, 

 at fir.-t to b tl.-st t -in d, ami 

 then to lie pu-h. (1 in toward 

 tin- middle of the sphere. 

 By tin 1 advance of the 

 iuvairinatii n tin- depr- 

 grows deeper and deeper, 

 while the cleavage-cavity In- 

 comes to the same drirnr 

 diminished in si/.e. Finally, 

 the inv,-i_L r in:itcd portion (tiur. 

 44 ik) comes in contact \vith 

 the iniH-r surface of the un- 

 in\ai:iii:ited portion (/.) of 

 the blast ula. and completely 

 obliterates the clea\ 



cavity. As a result th-ro has been foi-incd out of the hollow 

 sj.hcre with a single wall a cup-shaped germ with double wall- - 

 the gastrula. 



TheCfcvity Of thegafitrnla, which r suits from the invaLrination and 

 is not to be confounded with the cleavage-cavity which it has sup- 

 plant d, is the primitive intestine (archenteron) (ud), or the intestino- 

 body cavity (coelenteron). This opens to the outside through the 

 primitive mouth (mouth of the archenteron, blastopore) (u). 



Inasmuch as the names primitive intestine and primitive mouth 

 ini^lit ( usily give rise to erroneous conceptions, let it be remarked, in 

 order to preclude from the start such an event, that the cavity and 

 \te-rn.-il oj-enini; which arise by this first invagination are not 

 equivalent to the intestine and mouth of the adult animal. The 

 archenteron of the germ, it is true, furnislus the fundament for the 

 inte.-tinal tube, but there are also formed out of it a number of other 

 organs, the chief of which are the subsequently formed thoracic and 

 abdominal cavities. The future destination of the cavity will there- 

 fore be better expressed by the term u /'." finally, tho 

 primitive mouth is only an evanescent >tructure among vertebr 

 animal>; later it i- clo>ed and disappears without leaving a trace, wliil 

 the permanent or secondary mouth is an entirely new structure. 



The two cell-layers of the cup, which are continuous with each 

 other at the edge of the blastopore, are called the two primary 



