306 c. (). wHir.AiA.v. 



Stranges aui" {-^. Compare also 'r-f). This description is 

 in harmony with the investiijatioiis of Kupft'er (I'l), Kowalevsky 

 (88j, Bnllour (U), ami SchuUz (M5j. Oellacher (124) on the 

 otlier hand regards the hind end of tlie embryo, instead of the 

 fore end, as fixed, according to which, as His has remarked, the 

 embryo must lengthen forward by intussusception. 



A comparison of figs. 1, 2, and 3 with somewhat later 

 stages, figs. 4, 5, and 6, will show that the neurula of the 

 chick, or of the fish, belongs to the same type as that of 

 Clepsine. The embryonic rim in the Selachian egg appears first 

 in the form of a ring; but this ring is composed of two homo- 

 typical parts, as evinced by their progressive concrescence which 

 begins at the fore end and advances towards the hind end, 

 precisely as in Clepsine. In Clepsine there is a cephalic portion 

 in front of the primitive groove Q;. </r). The same condition is 

 seen also in the shark (fig. 2) and in the chick (fig. 3). The- 

 primitive groove in Clepsine is continuous with the blastopore, 

 or the anus of Rusconi (E,.). The same is true of the sharks, 

 but not of the chick. This discontinuity in the case of the 

 chick is, however, made easy to understand by what happens 

 in osseous fishes. Bring the two marginal lobes (c. l.) in the 

 Selachian egg (tig. ~) into close ap])osition, and a single lobe, like 

 what we see in Salmo (fig. 5, c. I.), is formed. This closing up of 

 the two lobes would obscure or interrupt the continuity between 

 the primitive groove and the blastopore. In the chick (figs. 

 3 and 6) this modification of the typical condition is carried 

 still farther, as Eauber, in his excellent paper, " Primitivrinne 

 und Urmund,''^ has made clear. Here the homotypical lialves 

 of the embryo are not only blended into a single lobe at their 

 posterior point of junction, but this lobe (c.L, fig. G) has lost its 

 marginal position. This latter fact is in harmony with the fact 

 that only a small part of the blastoporic rim is used in the formation 

 of the chick-embryo. Evidence of the original connection between 

 the ])rimitive groove and the blastopore is seen in the marginal 

 notch (" Eandkerbe,^' Eauber) which sometimes makes its 

 appearance in the edge of the blastopore, just behind the 

 primitive groove (fig. 6, m. n.). 



This interesting remnant of the ancestral condition was first 

 seen by Pander (126, PI. I, fig. 1). His (V./t^tW) li^s observed 

 the same in several cases, and so has Eauber who was the first to 

 interpret it as '' the hind end of the primitive groove" (liH). 

 This interpretation is indirectly supported by the tyi)ical relation 

 between the digestive tract and the neural canal, first made known 

 by Kowalevsky. The direct continuity between these two tubes 

 found in Mustelus and Acanthias (-r/V) by Kowalevsky, accord- 

 ing to a citation by Eauber ( ',^,'), has been conlirmcd by Balfour 



