540 PROCEEDINGS OF THE ACADEMY OF [DeC, 



further clown to the region of the l)lastoporo. Thus the large group of 

 mesoderm cells found later around the i)har3'nx arise, in large part at 

 least, from the second quartet cells. It is, however, quite possible that 

 some of the derivatives of 4:dr later wander into this region also. 



We have now followed the history of the mesoderm from its origin 

 until it forms a layer of cells about the embryo just beneath the ecto- 

 derm. We may leave the discussion and comparison with other forms 

 until after we have traced the history of the endoderm. 



The Extoblast. 



The alimentary canal of the polyclads has been derived in various 

 ways by different investigators. Vaillant (68) and Keferstein (68) 

 observed that at the time the embryo began to rotate within its mem- 

 branes there was a mass of roundish fat-like spheres on the interior. 

 Hallez (78 and 79) observed drops of an egg-white-like substance on 

 the inside of the embryo, surrounded by a one-cell layer of the alimen- 

 tary canal. The cellular elements of the canal he derived rather doul^t- 

 fully from the four small cells at the oral pole (macromeres). Selenka 

 (81c) came to a similar conclusion, i.e., that the entire canal arose from 

 the four small "primitive endoderm cells" (macromeres). These were 

 carried into the embryo by the pharyngeal invagination and rapidly 

 spread over the large yolk cells (fourth quartet). According to this 

 investigator these yolk cells break up without nuclear division into a 

 large number of yolk spherules and serve as food for the developing 

 endoderm. They give rise to no morphological structure of the embryo. 

 Selenka (81) says that as soon as "die Nahrungsdotterzellen in ein 

 Dutzend oder mehr ungleich grosse kernlose Kugeln zerfallen sind. 

 beginnen die vier Ur-Entodermzellen (macromeres) ihre Theilung und 

 Wanderung. Zunachst strecken sie sich in die Lange, entsenden 

 Ausliiufer und breiten sich auf den benachbarten Dotterkugeln aus." 



Goette (82) in Stylochus pillidium finds, as already mentioned, vipper 

 endoderm cells. From these and from the middle endoderm cells 

 (fourth quartet) small cells are separated, which at first contain some 

 of the fat-like yolk. This soon disappears in these cells and the)' 

 come to form a definite layer of endoderm. The small lower endoderm 

 cells (macromeres) also take part in forming this layer. Targe drops 

 of the homogeneous yolk substance separate from the large middle 

 endoderm cells and are gradually absorbed by the cells of the canal. 



Lang (84) in Discocoelis, it will be remembered, also finds an upper 

 endoderm as well as a lower and a middle layer. Like Selenka, he no 

 longer finds any nucleus in the large cells (foui-th quartet) after the 



