ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



side, its arms extending forward to the second cleavage plane. It occupies this 

 position throughout the whole of the cleavage, its substance becoming localized in 

 a number of large rounded cells. In the gastrulation these cells are inrolled along 

 the lateral-posterior borders of the blastopore, thus reducing the posterior portion 

 of the blastopore to a groove and rendering the whole blastopore pear-shaped. No 

 such appearance is found in Amphioxus or amphibians where the blastopore retains 

 its circular form until a late stage; this may be interpreted as due to the fact that 

 in these animals the mesoderm is not so largely developed at an early stage, but 

 it furnishes no satisfactory reason for supposing that the mesoderm is not formed 

 in corresponding positions in all three classes. We know that the neural plate and 

 the notochord come from similar regions in all three, and it is most unlikely that 

 the mesoderm arises froin wholly different regions. 



Hatsehek's account of the origin of the mesoderm of Ampliioxus shows some 

 important resemblances to what occurs among ascidians. He found that running 

 back on each side from the first appearing primitive segments was a mesodermal 

 fold which led to a pair of pole cells in the ventral (posterior) lip of the blastopore. 

 All recent investigators nave denied the existence of these pole cells, and there 

 can be little doubt that Hatschek was mistaken with regard to them. Even in the 

 ascidian there are, strictly speaking, no pole cells in this region, nor anywhere 

 else in the embryo. The cells which in the ascidian occupy the position assigned 

 by Hatschek to the pole cells are the posterior mesenchyme cells. These cells form 

 the middle of the crescent, and from them a band of mesoderm cells runs forward 

 on each side, but these bands were not formed by the teloblastic growth of the pos- 

 terior cells; on the contrary, their substance was localized in the crescent before 

 cleavage began. However the non-existence of the pole cells of Ampliioxus does 

 not destroy belief in Hatsehek's account of the mesodermal folds which run back- 

 ward from the primitive segments to the blastopore. Several investigators have 

 recognized such folds or bands, and their existence can scarcely be doubted. These 

 bands have been seen only in older gastrulse, and they here occupy a position 

 which corresponds very closely with the mesenchyme bands in the ascidian gas- 

 trula. The separation of the muscle band from the mesenchyme band in the older 

 gastrulse of the ascidian [v. p. 69) is evidently a coenogenetic phenomenon, since 

 nothing of this sort is known to occur elsewhere. If the mesodermal bands of 

 Ampliioxus are present in earlier stages than those in which they have been repre- 

 sented by Hatschek, and if they occupy the same relative position as in the asci- 

 dian they would surround the posterior border of the blastopore, and only by over- 

 growth of the dorsal lip and the narrowing of the whole blastopore would they 

 come to lie alongside of the notochord. That mesodermal cells are present in the 

 posterior lip of the gastrula of Amphioxus at an early stage is made probable by 

 the observations of Lwoff, Klaatsch, Morgan and Hazen. Lwoff has found that 

 the longitudinal musculature of Amphioxus arises along the hinder lateral parts of 

 the blastopore, where it comes from ectodermal cells, as he thinks, which are in- 

 rolled. Klaatsch agrees with this and compares the "pole cell bands" of ascidians 



