DEVELOPMENT OF THE MEDULLARY TUBE, ETC. 539 



end coinciding with the posterior edge of the blastopore. The 

 definitive principal axis thus forms an acute angle with the primary 

 axis. The blastopore has undergone shifting to the dorsal side of 

 the embryo ; it now gradually decreases in size, chiefly through a 

 backward growth of its dorsal or anterior border. The posterior 

 (ventral) edge of the blastopore, on the contrary, remains stationary 

 during this narrowing process. It is always marked by two larger 

 entodenn-cells lying symmetrically to the median plane (Fig. 280 C) ; 

 these are claimed by HATSCHEK as pole-cells of the mesoderm.* 

 Finally, the embryo becomes more elongated. The ventral surface 

 is distinguished by being arched, while the dorsal side, which was 

 originally occupied by the blastopore, is distinctly flattened. The 

 posterior end of the dorsal side is occupied by the now very small 

 vestige of the blastopore (Fig. 280 C). Even at this stage, the 

 external surface of the embryo is covered with short flagella which 

 enable it to rotate within the egg-envelope. 



In the position of the blastopore and the conditions under which it closes, 

 there is close resemblance between Amphioxus and the Ascidia (cf. p. 342). 



In our description of the transformations undergone by the gastrula-stage, 

 we have for the most part followed HATSCHEK. A rather divergent account 

 has recently been given by LWOFF (No. 17) who, in dealing with the closure 

 of the blastopore, lays the chief stress on the independent and rapid growth 

 of the ectoderm at its dorsal margin. (Stress should also be laid on the 

 protrusion of the lateral edges of the blastopore, a point which was overlooked 

 by LWOFF). From this rapidly growing dorsal edge, ectodermal elements are 

 said to be invaginated dorsally into the gastrula-cavity and there finally to 

 force the proper entodermal elements from the dorsal to the ventral side of 

 the cavity. According to this view, the cells forming the dorsal wall of the 

 archenteron are ectodermal and it is from this cell-layer that LWOFF derives 

 the chorda. LWOFF was not able to convince himself of the presence of the 

 primitive cells of the mesoderm which HATSCHEK found marking the posterior 

 end of the longitudinal axis of the embryo. 



B. Development of the Medullary Tube, the Primitive Segments 

 and the Notochord. 



The next ontogenetic stages of Am/tkwxtu are characterised by the 

 increase in length of the whole body. The median part of the dorsal 

 surface, at the same time, sinks in somewhat (Fig. 281), and this 



* [SOBOTTA (No. X.) agrees with LWOFF (No. 17) in denying that the blasto- 

 pore closes from before backward. They believe that it gradually diminished, 

 on all sides. SOBOTTA was unable to find the pole-cells and does not believe' 

 that they exist, at any rate at the early stages figured by HATSCHEK. ED.] 



