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ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 85 
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 Amphzoxus or amphibians where the blastopore retains 
its cireular 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 from wholly different regions. 
Hatschek's account of the origin of the mesoderm of Amphioxus 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 have 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 Amphioxus does 
not destroy belief іп Hatschek'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 gastrule, and they here occupy а 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 
gastrule of the ascidian (о. р. 69) is evidently a coenogenetie phenomenon, since 
nothing of this sort is known to occur elsewhere. If the mesodermal bands of 
Amphioxus 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 Amphzoxus at an early stage is made probable by 
the observations of Lwoff, Klaatsch, Morgan and Hazen. Lwoff has found that 
the longitudinal musculature of Amphzoxus 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 
