688 THE OVUM AND BLASTODERM. 



to the same conclusion as those of Balfour, and if confirmed would go far to 

 prove the occuiTcnce of a secondary or jirolong-ed segmentation in the subger- 

 minal j-olk, to which allusion was previously made. 



It is'right. however, to state that on the other side there is the weighty autho- 

 rity of KoUikcr. T\-ho. in association with the younger Vii'chow, has recently 

 soiight in vain for the evidence of such migration as has been described by 

 the observers previously referred to. and attributes the formation of the meso- 

 blast enth-ely to the proliferation of cells connected originally with the lower 

 surface of the epiblast. 



Difference in Animals. — The foregoing description applies to the sj-mmetrical 

 position and central mode of development of the blastoderm which belong to 

 the ova of reptiles, buxls and mammals : but it Ls right to state that in the lower 

 vertebrata, or in amphibia and in osseous and cartilaginous fishes, there are 

 several remarkable differences. Among these may be particularlj' noticed the 

 non-symmetrical development of the blastodenn, and the excentric position of 

 the commencement of the embryo : the involution of the epiblast at the aper- 

 ture of the blastodenn termed ' anus " by Rusconi. or blastopore, at which the 

 cells of the epiblast become continuous with the larger deeper cells from which 

 the mesoblast and hypoblast originate. (See F. M. Balfour, " On the Develop- 

 ment of the Elasmobranch Fishes,"" and " Comparison of the Development of 

 Vertebrates,"" in the Quart. Joum. of Jlicroscop. Science for Oct. 1874. and July, 

 1875 ; E. Eay Lankester. '• On the Primitive Cell Layers of the Embryo." Sec, in 

 the Ann. and Jlag. of Nat. Hist, for 1871. 



The Blastoderm of Mammals. — A variety of observations have shown that 

 the blastodenn of mammals consists, when fully fonned. essentially of the same 

 kinds of elements arranged in three layers, as previously described in birds ; 

 but the mode of fonnation of these layers has not yet been fully investigated. 

 By the observations of Bischoff. Coste, and Reichert, it was ascertained that as 

 the result of the first segmentation the yolk became invested with a comjilete 

 superficial covering of distinct nucleated cells, which may be looked upon as 

 coiTesponding to the ui)i)er or oiiter layer, or epiblast. "Within this there remains 

 for a time a thick plate or rounded mass at one side of opaque spherules, 

 which seemed to be segment spherules not yet converted into cells, and the 

 interior of the yolk was elsewhere filled with a graniilar fluid. Some time later, 

 or about the fifth day in the rabbit"s ovum, a thickened spot, the germinal area 

 of Bischoff. or tache embiyonnau-e of Coste, gi'adually made its appearance in 

 the place where the primitive trace of the embryo is afterwards formed. This 

 consisted in a thickening of the layer already fonned. and of an accumulation 

 of a layer of new cells below it. which, gradually extending itself over the 

 sui'face of the yolk, gives a second covering of cells to the ^vhole. 



In a carefully conducted series of recent obsez'vations, Hensen finds (Zeitsch. 

 filr Anat. u. Entwick. Leipz. Xov. 187o) that in the rabbit"s ovum, at the time 

 when the germinal disc is still round {'> days 4 hours) the epiblast. with its 

 central thickening, fonns a complete A-esicular covering of the yolk, but that the 

 hypoblast, lying belo'«- the disc, does not extend over more than a thii-d of the 

 circimiference. Tlie cells of the middle layer are at this time restricted to the 

 liinder part of the genu disc, in which place the primitive trace of the embryo 

 first appears. Kolliker also, in the same animal (Verhandl. d, Physik. Med. 

 Gesellsch. z. "Wiirzbiu-g, Kov.. 187."j) describes the inner layer (h\']3oblast) as 

 spreading rapidly over the inner surface of the oriter layer or epiblast, so as at 

 last to give a complete double covering to the yolk. 



In sections of a vesicular blastoderm of the cat, prepared by Mr. Schafer, but 

 not yet described, two layers may. as he has pointed out to me. be seen, the outer 

 of which (epiblast) lies immediately within the primitive chorion and is co- 

 extensive with it, whilst the inner layer (hypoblast), although also complete, 

 forms a smaller ring than the outer, and is in contact with the latter at one place 

 onl3'. Both layers, although elsewhere fonned of a single stratum of cells, are 

 here slightly thickened, but especially the outer (as if a mesoblast were about to 

 be developed from it) : the hypoblast at this place appeai-s bounded superficially 

 by a delicate cuticular film. 



