ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 735 



G. septentrionalis is an exception to the rule that the female Cope- 

 pods continue to carry their eggs till the Nauplius-stage is reached ; 

 the eggs are laid in the water, just as in the case oiDias longiremis and 

 some other Calanidee. When laid, the egg is spherical, its yolk consists 

 of a granular protoplasm, in which are found albuminous bodies ; at first 

 there is no polar differentiation. The author enters into an account of 

 the characters of the investing membrane ; and proceeds from this 

 to a history of the ovarian nucleus, and to note the presence of but a 

 single sperm-nucleus. Cleavage is complete, and there early appears 

 a small spindle-shaped cleavage cavity in the centre of the egg. 

 When 32 blastomeres have been developed there is a large central 

 cell, which is distinguished by its rich supply of yolk-spheres, and 

 its coarsely granular protoplasm. This is the " central endodermal 

 cell ; " other endodermal cells are the single anterior and the four 

 lateral cells. In the hinder region mesodermal elements are already 

 apparent. There is a considerable pause after the 32-cell stage, 

 during which the blastomeres appear to absorb yolk-material. Two 

 primitive mesoderm-cells are to be seen at a later period, which, by 

 dividing, give rise to four, in very much the same manner as that 

 already observed by Eabl in the Molluscan Planorhis. 



The endodermal invagination takes place thus : the eight lateral 

 endodermal cells sink inwards, they are followed by the four central 

 and by the uprising of part of the ectoderm to give rise to two 

 ridges at the sides of the blastopore. The four mesodermal cells 

 now lie between the ectoderm and the invaginated endoderm ; and 

 the ectodermal cells undergo further division. This is followed by 

 the closing of the blastopore, the characteristics of which are pointed 

 out and explained. Only two stages could be observed between this 

 point in development and the appearance of the Nauplius, which, 

 when complete, has an oval form, the first antenna uniramose, the 

 second and the mandible biramose ; the oesophagus is short, the mid- 

 gut straight, but the rectum and anus are not yet present and are 

 only represented by an ectodermal elevation. There is a supra- 

 cesophageal ganglion, and a faintly reddish pigment-spot ; the sub- 

 CBSophageal ganglion belongs to the mandibular segment ; all the 

 parts of the nervous system are still in connection with the integu- 

 ment. 



In its further development the Nauplius undergoes a number of 

 ecdyses, in the course of which the body elongates, and the hinder 

 end becomes provided with a number of hooks ; these are described 

 in some detail. 



The author is led to some general conclusions which may here be 

 shortly summarized. He thinks that probably the ejection of the 

 " directive corpuscles " is not a rare phenomenon among the Crustacea ; 

 but it does not seem to obtain in those forms which exhibit 

 parthenogenesis. He believes that, notwithstanding some appear- 

 ances to the contrary, cleavage is really bilaterally symmetrical ; the 

 development of the mesoderm from two cells appears to be very 

 common among the Entomostraca. In addition to the significant 

 pause in development already mentioned, another occurs at the time 



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