GASTRULATION AND EARLIEST DEVELOPMENT 197 



In a slightly further advanced stage (fig. 40d) the greater 

 part of the slit-like blastopore has been overgrown by the 

 medullary folds, only at the hindmost extremity is there 

 still a little opening (bl.) from which the anal groove runs to 

 the anal pit (a). This anal groove, with a deeper impression 

 at its anterior (rest blastopore) and at its posterior end 

 (anal pit) appears to have been confused by several authors 

 with the slit-like blastopore of a somewhat earlier stage 

 (fig. 40 a and b). They accordingly imagine this slit to 

 have closed in the middle by coalescence of the opposite 

 borders, leaving only a passage at the anterior and at the 

 rear end, these being the future neurenteric canal and the 

 anus. The rudiment of the tail is claimed to arise as a 

 double knob at the right and the left side of the line of 

 coalescence, these knobs fusing afterwards over the middle 

 of the blastopore. Thus ZlEGLER (1892), in his note on 

 the surface views of R ana-embryos, writes: "Etwas spater 

 sieht man an Stelle des Spaltes eine Rinne, welche vorn 

 in den Canalis neurentericus, hinten in die Aftergrube 

 iibergeht; es sind namlich jetzt die seitlichen Blastoporus- 

 lippen median zur Vereiriigung gekommen". The same 

 views are put forward by HertwiG in his Lehrbuch. Even 

 a close examination of surface views, however, teaches 

 us that the anal groove is by no means identical with the 

 slit-like blastopore but that its anterior end coincides -with 

 the rear end of the latter. In fig. 40 c we see it already 

 running from the slit-like blastopore to the anal groove. 

 The study of median sections precludes every possibility 

 of doubt. These sections invariably give the condition of 

 fig. 4 (plate) which links up with fig. 3. The anus is on 

 the point of breaking through, the blastopore of being 

 closed by the medullary folds. The two are quite 

 independent. 



The step to fig. 5 (plate III) is again a small one. In fig. 3 

 we see the cerebral plate already curving in Especially notic- 

 able is the opposition between the praechordal cerebral 

 plate and the epichordal medullary plate which, as a matter of 

 fact, is in this stage no longer a flat plate but curved into a 

 groove between the medulla) y folds. Fig. 3, therefore, is reali- 

 zed only in one or two sections which are exactly median; to 

 the right or the left side immediately one of the medullary 

 folds is intersected, as indicated in fig. 3 with a dotted 

 line. A paramedian section in this series thus offers a much 



