A Contribut. to the Embryol., Life-history, anrì Classificat. of the Dicyeraids. 7;^ 



fail to see what better evidence of the existence of such a lamella coiild 

 be brought than has already beeii produced by Julin ; and I can not 

 therefore accept the Interpretation which be has given to the »cellules 

 intermediaires«. On this point Julin remarks, that »we find in 

 certaiu Metazoa a fonnation quite similar to that of the muscular layer 

 of the Orthonectidae, — a forniation which is, however, completely in- 

 dependent of that of the mesoderni and has nothing to do with the latter. 

 The muscles of the Orthonectidae consist only in a 

 histological differentiation of the superficial layer 

 of the endoderm. It is something analogons to what takes place, 

 according to the observations of the Hertwigs , in the forraation of the 

 muscular tissue of the Actiniae. Here also, 'es sind allein die epithe- 

 lialen Begrenzungsschichten , aus welchen sich die für die höhere Ent- 

 wicklung des Organismus so überaus wichtigen Elemente, die Muskel-, 

 Nerven- und Sinneszellen differenziren' (34) , and this formation has 

 nothing in common with the mesoderm, which consists of a mesenchym 

 composed solely of a gelatinous substance«. (21, p. 42.) Then comes 

 the following important Statements. ))La seule ditférence, qui existe 

 entre les éléments musculaires des Actinies et ceux des Orthon ectides, 

 consiste en ce fait que, tandis que chez les premiers ces éléments restent, 

 pendant toute la durée de la vie, en continuité directe avec les cellules 

 endodermiques qui leur ont donne uaissance, chez les seconds , au con- 

 traire, ces éléments se distinguent des autres cellules en- 

 dodermiques à une phase plus ou moins reculée du de- 

 ve loppe ment. Cette différenciation d'une partie des cellules endo- 

 dermiques en cellules musculaires se fait beaucoup plus tard chez la 

 femelle que chez le male de Bh. GiardiÌA< (p. 42 — 43.) 



To my mind, Julin has shown conclusively that the Orthonectidae 

 bave a veritable mesoderm, — a distinct cell-layer, arising between 

 the entoderm and ectoderm at an early period of the development, not 

 as the result of a histological differentiation, but prior to and inde- 

 pendent of such differentiation. In the male, tliese cells arise at the 

 two poles of the endodermic celi ; and the fact that they do not form a 

 complete envelope around the centrai celi or cells until after they bave 

 undergone a fibrous differentiation is of course no objection whatever to 

 the view I bave taken. In the case of the female, the mesoderm (»couche 

 upertìcielle de l'endoderme < , Julin) forms a complete envelope around 

 the endoderm («masse endodermique centrale«, Julin), and this long 

 before the appearance of any muscular differentiation. To affirm that 

 this cell-layer is not a mesoderm, and at the same time maiiitain that 



