ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 69 



In this process the ectoderm does not, for some time, close up the notch at the pos- 

 terior end of the blastopore (fig. 52); this is one of the last steps in the process 

 of closure. 



The overgrowth of the anterior lip continues until it has covered about three- 

 quarters of the dorsal face; meanwhile the animal pole is shifted nearer to the 

 point of greatest curvature at the anterior end. and the blastopore is transported 

 from the dorsal side toward the posterior end. In this process the rows of muscle 

 cells which at an earlier stage stretched from the posterior pole to the second 

 cleavage plane, and were antero-posterior in direction, are tilted up at their anterior 

 ends and pushed backwards until they lie at the hinder end of the embryo and run 

 in a dorso-ventral direction (figs. 52, 53, 50, 157). This complete change in the 

 direction of the rows of muscle cells I found most perplexing and difficult to under- 

 stand. In early stages the crescent, and the mesoderm cells which form from it. lies 

 just below the equator of the egg, and in the anteroposterior plane; in these later 

 stages the muscle cells are transverse to the anterior-posterior axis. A detailed 

 study of intermediate stages shows how this change is brought about. After the 

 closure of the anterior part of the blastopore, corresponding to the bar of the T. 

 the anterior lip does not overgrow the blastopore groove (stem of the T) and its 

 lateral walls, which are composed of the muscle cells; on the other hand, these 

 lateral walls lie at a higher level than the anterior lip, and the continued growth 

 of this lip pushes the muscle cells and the groove before it. As the posterior lip 

 remains stationary during this process it happens that the entire dorsal portion of 

 the posterior quadrants is tilted up in front and pushed backward until it forms the 

 posterior end of the embryo, the posterior lip becoming vental and the anterior lip 

 dorsal in position. Thus the blastopore groove, which lay on the dorsal side pos- 

 terior to the middle, comes to lie at the* posterior end of the embryo and the walls 

 of the groove, containing the muscle cells, come to be terminal in position and ver- 

 tical in direction (figs. 50-53). The mesenchyme cells, as well as the caudal 

 endoderm. lie at so low a level that they are not disturbed by the overgrowth of the 

 anterior lip, consequently the rows of these cells still preserve an antero-posterior 

 direction (fig. 157). Thus the mesenchyme and muscle cells, which in earlier 

 stages lay side by side, come to be separated anteriorly, and only remain in con- 

 tact with one another at the hinder end of the strand of caudal endoderm cells; 

 the mesenchyme cells in this region are derived from the median part of the cres- 

 cent and they ultimately become separated from the remaining portion of the 

 mesenchyme which comes to lie in the trunk (figs. 161-167). 



These general changes in the shape of the embryo at this stage are accom- 

 panied by divisions of many of the cells, some of which we may now consider. In 

 all the ectoderm cells the ninth cleavage is nearly synchronous ; in the posterior 

 quadrants the spindles are approximately antero-posterior in direction, and the 

 same is true for the two hindmost rows of the anterior quadrants, but in most 

 of the other cells of the anterior quadrants the spindles are transverse, thus it 

 happens that the animal pole is shifted forward (fig. 149). As compared with 



