ON THE PROCESS OF GASTßULATIOX IX CHELONlA. 243 



7 meiner oben zitierten Mittheilung (No. 1<S of my list. Same as 

 Fig. 176 of No. :21) auch geeignet ist, die ^"erhältni8."se hei der 

 Schildkröte zu illustriren. Es treten zunächst einige wenige isolirte 

 Durchbrechungen der untern Urdarmwand (nebst dem unter derselljen 

 wegziehenden Dotterblatt) ein; indem sodann beständig neue Lücken 

 auftreten, die alten sich aber vergrüssern gelangt man zu Stadien, Ijei 

 den von der gesammten unterer Urdarmwand nur noch ein unregel- 

 massiges, bei den verschiedenen Embryonen verschieden gestaltes 

 System von Netzbalken erhalten geblieben ist. Schliesslich kommen 

 auch diese letzten Reste zum Schwunde, wodurch dann das bisherige 

 Urdarmlumen mit dem subembryonalen Kaum zusanunenfiiesst." 

 After seeing his Figs, bba and h (No. 21) it seems no longer possible 

 to doubt the great anterior extension of the archenteron in Gecko, 

 bef<jre it breaks open. As t(j this point in Chelonia I shall reserve my 

 judgment until his prijmised full paper on the tort(3ise appears. 



Van l^eueden (No. 13) describes in Mammalia two kinds uf 

 openings by whicli the cliorda-canal comes to open below into the 

 blastoderm cavity, viz: — (1) an anterior transverse slit, and ('2) several 

 openings which soon run together into a. single posterior longitudinal 

 slit. From what has been stated aljo\e. J need hardly say tliat 1 do 

 not tind any such differentiation of openings in Chelonia. 



The changes that follow on the ijreaking through of the 

 invagination-cavity can best be seen in the surface views. Figs. 

 5 and oa — (S and 8c( are introduced to illustrate this point, ^\'e 

 have seen how the primitive knob, at first c<jnfined to a small 

 accumulation of cells at the posteri(jr edge of the embryonic shield 

 (Fig. y), gradually spreads itself anteriorly until it comes to occu])y 

 quite a considerable area on the ventral surface of the embryonic 

 shield (Figs. 13- IG), when the invagination cavity breaks through be- 



