74 C. 0. Whitman 



a g-elatiüous lamella eutirely devoid of cells is a mesoderm, is the 

 somewhat singular position now occupied by Van Beneden and Julin. 

 That Julin's denial of a mesoderm is not easily reconciled with bis 

 account of the origin of thc muscle-fibres , has , if I mistake not , been 

 quickly perceived by Van Beneden. My ground for so iuferring is the 

 fact that Van Beneden has not allowed these Statements to pass with- 

 out a demurrer. »)I1 est à remarquer, « says Van Beneden, »qiie rien 

 dans les recherches de M. Julin, ne prouve que cette couclie cellulaire 

 engendre les fibrilles , loin d'établir qu'elle est tout entière employée à 



la forniation de ces fìbrilles Si réellement, ce qui ne me paraìt 



pas prouvé. ces fibrilles sont des dépendances de ces cellules, il me 

 semble bien plus probable que celles-ci , après avoir donne nalssance 

 aux fibrilles et avoir affecté temporairement le type épithélio-musculaire, 

 se transforment en oeufs tout comme les cellules axiales.« (2, p. 221.i 



According to this vìcav, the endoderm of the Orthonectidae differen- 

 tiates early into two distinct parts , a centrai mass of cells and 

 a superficial ce 11-1 ay er iuterposed between the centrai mass and 

 the ectoderm. The entire centrai portiou is converted into sexual prod- 

 ucts: the superficial layer imitates for a short time an epithelio- 

 muscular layer; but it maintains this character only transiently, 

 for its epithelial portion soon becomes sexual cells, and the persistent 

 muscular portion thus becomes an independent layer. Julin's view 

 differs from this only in one point : he holds that the entire superficial 

 layer is converted into the muscular layer. Van Beneden fiuds it diffi- 

 cult to accept Julin's conclusion in this particular, since the muscular 

 layer is exceedingly thin, while the superficial layer itself is very thick. 

 »How can we admit,« he asks, »that a layer so thick as the range of cells 

 subjacent to the epidermis of larvae like those which are figured fPl. ?», 

 figs. 8— lo. Julin , is whoUy employed in the formation of these fibrils?« 



For my part , 1 see nothing incredible in the opinion that this en- 

 tire superficial layer, after becoming gradually thinner by a process of 

 atrophy, should eventually take the form of a thin muscular layer. It is 

 of course possible that only a part of its cells assume the muscular form, 

 the rest becoming sexiial products. Either of these views seems to 

 me more probable than the ingenious theory of a transient epithelio- 

 rauscuiar stage. The assumption of the reproductive function by a part 

 of these cells is a possibility which recalls our attention to a point inci- 

 dentally mentioned before. I have compared the two primary germ-cells 

 in the vermiform embryo of a Dicyemid to the two primary cells that 

 arise at the poles of the centrai celi of the male Orthouectid. 



