76 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



to the cephalic pole of the embryo; and this view has been supported by 

 Garbowski (1898); on the other hand. Kowalevsky (1867) and many recent 

 writers on the development of Amphioxus (Lwoff 1894, Klaatsch 1896, Samassa 

 1898, Morgan and Hazen 1900). have considered that the most highly arched 

 portion of the late gastrula represents the animal pole. Since this point is said 

 to become the anterior end of the embryo it is evident that according to this 

 view the chief axis of the egg coincides with the chief axis of the embryo and 

 is antero-posterior in direction, whereas in ascidians it has heretofore been claimed 

 that the egg axis is dorso-ventral in direction and hence perpendicular to the 

 same axis in Amphioxus. 



Sueh diversity in this most fundamental of all axial relations seems very 

 improbable considering the many points of resemblance between these groups, 

 and at least sueh conflicting results should be supported by the best of evidence 

 before being given general credence. 



Korschelt and Heider in their excellent text book attempt to harmonize these 

 differences in axial relations between Amphioxus and the ascidians by regarding 

 the anterior pole of the ascidian gastrula as the animal pole, but I agree with 

 Samassa (1894). and Castle (1896), that the animal pole never comes to lie at the 

 anterior end of the embryo, though unlike them I hold that it does move in that 

 direction. 



In Amphioxus as in the ascidians the anterior limit of the neural plate is 

 situated some distance behind the most highly arched portion of the gastrula, 

 and even if the latter be regarded as the animal pole it would still be true of 

 Amphioxus as of the ascidians that the neural plate does not reach as far forward 

 as the animal pole. But there are reasons for thinking that the animal pole lies 

 ventral to the most highly arched portion of the Amphioxus gastrula. Many inves- 

 tigators agree that the animal pole lies opposite the blastopore ; Samassa has observed 

 in a small percentage of eggs that the polar body is still attached to the embryo at 

 a time when the blastopore is growing smaller, and in all sueh cases he found it at 

 t be pole opposite the blastopore (although, as he maintains, at the anterior end of 

 the embryo). But the point opposite the blastopore lies ventral to the most highly 

 arched portion of the embryo. Even if it should be assumed that both ventral and 

 dorsal lips grow equally, the animal pole would still be located on the ventral side 

 of the most highly arched portion, owing to the peculiar shape of the embryo; if 

 the dorsal lip grows more rapidly than the ventral, which in the light of what 

 takes place in ascidians and amphibians seems probable, the animal pole must lie 

 still farther toward the ventral side. In any event a considerable space must inter- 

 vene between the anterior limit of the neural plate and the animal pole. 



The work of Garbowski (1898), shows that the longitudinal axis of the larva 

 of Amphioxus forms an angle of about 70 with the gastrular axis. a result which, 

 like that of Eatschek and Sobotta, agrees very closely with my observations on 

 ascidians. and which practically removes the supposed discrepencies in axial rela- 

 tions between these two classes. 



