hargitt: pennaria tiarella and tubularia crocea. 201 



gonophore; but there seems also to be a tendency to an abbreviation 

 in this stage of the development. This abbreviation is most evident 

 when the cleavage cavity is greatly reduced or lacking, in which case 

 cleavage and germ-layer formation may not be sharply separated from 

 each other, as Wulfert (:02) also found in Gonothyraea. 



By a division of the cells of the blastula, the cleavage cavity becomes 

 entirely filled with cells. This happens by the process designated by 

 INIetschnikoif ('86) as primary delamination of the multipolar sort. 

 The same thing was found to occur in Tubularia mesembryanthemum 

 by Brauer ('91 ) and by other workers on other Hydromedusae. 

 Since a blastula stage does occur, this part of the development clearly 

 belongs to the germ-layer formation, the inner cells representing pri- 

 mary entoderm and those at the surface primary ectoderm; a true 

 morula, therefore, does not occur. The definitive ectoderm and 

 entoderm are formed by a further division, specialization, and read- 

 justment of the primary ectoderm and entoderm cells, and the eventual 

 formation of a supporting lamella between them. 



This interpretation is not at all inconsistent with the syncytial 

 conditions described by Hickson ('90, '94), Hargitt (:04^, :04''), and 

 Brooks and Rittenhouse (:07). Indeed, from the figures of the last 

 two authors it appears that the syncytium represents the so-called 

 morula, and that an earlier primary delamination has occurred. 

 Their figures also show between the cells spaces, more or less connected, 

 which are indications of a reduced segmentation cavity. The very 

 great delay in the formation of definite cells in Eudendrium (Hargitt, 

 :04'^) and the Hydrocorallinae (Hickson, 1. c.) is due, as these authors 

 state, to special conditions of development, viz: the presence of yolk, 

 and development within a very narrow cavity, from which the embryo 

 must later migrate. 



6. Double nuclei. — Multivesicular nuclei have been found in 

 many animals, chiefly in the re-formation of the daughter nuclei after 

 mitosis. Often each chromosome occupies at first a separate vesicle, 

 but these commonly fuse into a single vesicle, which is characteristic 

 of the resting condition. Such nuclei have been found in Hydro- 

 medusae by Metschnikott' ('82), Wulfert (:02), and Hargitt (:04«)". 



Distinctly double nuclei, similar to those found in the cells of the 

 blastula and later stages in Tubularia, have been less often described. 

 In Coelenterata only Conklin (:08) found sometimes in Linerges in 

 the first division such double vesicles, which he thinks represent the 

 halves of egg and sperm nuclei. Hacker ('95) and Ruckert ('95) 

 were the first to describe such nuclei, finding them in Copepoda. 



