78 RECORD OF CURRENT RESEARCHES RELATING TO 



cuticle would considerably interfere with its diffusion into the body of 

 the embryo. 



Comiug to the stage in which there are two thoracic segments 

 developed, we find we have a creature in which the two pairs of 

 antennfe and the inaudibles are well developed, but as yet there 

 are no maxillary appendages. The frontal plate consists of two 

 layers, and is divided into two portions ; the CBSophagus still ends 

 blindly. The genital segment has widened out transversely, and 

 there is evidence of a constriction in the median line. Four pairs 

 of thoracic appendages become developed before the maxillae appear, 

 as indeed does the retina of the compound eye, and the cerebral 

 portions develoj)ed from the frontal plate ; the genital rudiment is 

 divided into two distinct portions, which proceed to take up a more 

 lateral position. With the appearance of the knob which forms the 

 rudiment of the first maxilla, the four anterior pairs of thoracic 

 appendages become differentiated into an outer and an inner ramus, 

 and the branchial saccule begins to be developed. The ganglionic 

 cord becomes laid down in the two lateral thickenings of ectoderm 

 which lie on either side of the primitive groove. The author then 

 shows that the shell-gland is mesodermal in origin, and after 

 describing the separation of the central nervous system from the 

 ectoderm and the metamorphoses undergone by the mouth-organs, 

 comes to the point at which the heart is first apparent. Henceforward 

 the changes that take place are those in which the creature assumes 

 more and more its adult characters. 



In some " theoretical considerations " the author draws atten- 

 tion to the characters of the animal pole, and points out that there is 

 evidence of a polar differentiation in the ovum ; this is, of course, to 

 be associated with the superficial mode of cleavage, but it is of 

 importance as being a qualitative difference, and not merely a 

 quantitative difference, such as is expressed in the " jjolar arrange- 

 ment " of the yolk. Turning to the arrangement of the genital 

 cells, the author is not in agreement with Gegenbaur in regarding 

 the generative apparatus as being primitively single ; he supports 

 his position by a reference to Brancliipus, " the oldest Phyllopod," 

 where, as Glaus has shown, the generative apparatus is paired, and 

 by the consideration that the mesodermal bands are themselves primi- 

 tively paired ; when they are not so, and when the genital cell arises 

 singly, it is because of the changes necessitated by a superficial mode 

 of cleavage, so that the phenomena are consequently merely secondary. 



The body of Moina may be divided into a cephalic segment, a 

 number (eight) of thoracic segments, and a terminal one ; to the first 

 belong the antenna? ; the mandibular segment is the first of the 

 thoracic ; and appendages are never developed on the last of all. The 

 question of the presence of a trochosphere stage * is thus resolved ; 

 there is, of course, no double circlet of cilia, but all the other organs, 

 with the exception of the head-kidneys, are certainly present. In 

 that the shell-gland arises from the mesoderm, it resembles the 

 segmental organs (nephridia) of the Annulata. 



* Hatschek: see this Jomual, ii. (1879) p. 5G6. 



