INVEETEBEATA, CEYPTOGAMIA, MICEOSCOPY, ETC. 403 



of silver ; these were brought at once into a } per cent, solution, and 

 after remaining in it from one-half to two minutes, according to their 

 age, were immersed in distilled water, and exposed to the light. If 

 more than four days old they were opened by means of fine needles 

 and the envelopes carefully removed. The preparations were then 

 treated with picrocarmine, eosin, or other colouring matters, and 

 mounted in glycerine or balsam. Sections wore cut in a manner which 

 is described. 



The periods of development are divided into nine stages, the first 

 of which is the metagastrula. The embryo under description was taken 

 from the uterus seventy hours after the coitus,- and exhibited the 

 following dimensions : — 



Diameter of the metagastrula .. .. 0'09 mm. 



Mean thickness of the ectoderm .. 0*018 mm. 

 Diameter of the endodermic mass 0*052 to 0" 06 mm. 



Thickness of the zona pellucida .. • 015 to • 016 mm. 



The egg was completely spherical ; the ectodermal cells were all 

 convex outwards ; some spermatozoids could be made oi\t, but the 

 directive globules seemed to have completely disappeared. In optical 

 section forty ectodermal cells could bo counted ; their form, really 

 polyhedral, appeared to be cuboid, and their delimitations were with 

 difiiculty discernible. The limits between these cells and those of 

 tlie endoderm were, however, very well marked ; the former are 

 also both clearer and more finely granulated, notwithstanding the 

 fact that larger granulations were also visible. The endodermal 

 mass had an almost ovoid form, and it filled completely the cavity 

 fonued for it by the ectoderm ; more opaque than the outer layer, 

 it takes a brownish tint under the action of osmic acid, which also 

 brings into stronger relief the boundaries of its constituent cells. 

 The cells are jjolyhedral, and those which take part in the formation 

 of the region of the blastoi^ore are elongated in the direction of the 

 axis of the embryo ; thoy arc larger and more granular than the ecto- 

 dermal cells, and the granulations are more equally disi)osed through 

 their protoplasm. When the endodermal mass is isolated, the nuclei 

 of its cells, which are large, and provided with several nucleoli, 

 become mucli more distinct. 



It is impossible to follow the author through his detailed account 

 of all the diil'erent stage-! witli equal minuteness. Attention may bo 

 more es[)C'ciully directed to the conclusions at which he arrives. At 

 the end of the segmentation jieriod, at the moment when the ovum 

 penetrates into the uterus, the embryo is formed of two distinct cell- 

 layers, the ecto- and endo-dermal. At this moment thei'o is a solution 

 of continuity in the ectoderm, which forms the blastopore ; in other 

 words, the embryo is a gastrula, whicli is formed during the segmenta- 

 tion by epiboly. To this emluyonic stage the name of metagastrula 

 is applied. The blastopore goiiurally closes a short time after the 

 ovum passes into the uterus, and before the development of the blasto- 

 dermic cavity ; the position of this orifice appears to be relatively 

 cxcentric to the future gastrodiscus, and when it becomes dosed no 



