256 SUMMARY OF CURRENT RESEARCHES RELATING TO 



substance, consisting of blastoderm and lecithoblast, and tbe unorganised 

 substance in yolk. As lecithoblast be includes all germ-material, 

 wbetber in tbe form of cells, or in a syncytium, which is laden with yolk, 

 but takes no direct part in tbe formation of the germ-layers or the 

 primitive embryonic organs. Such are tbe periblast of Teleosteans and 

 Selachians, the germ-wall of bird and reptile eggs, the cellular contents 

 of the yolk-sac in the reptilian egg, and the so-called yolk-nuclei of 

 Amphibian eggs. In mammals tbe lecitboblast is functionally replaced 

 by the trophoblast. In the fowl the germ-wall (yolk-sac wall) consists of 

 cells which ramify through the outer layer of yolk and absorb it. The 

 periblast of Selachians is formed by the union of the blastomeres lying 

 next tbe yolk ; tbey take up a large amount of yolk, and the cell-bound- 

 aries disappear so that a syncytium is formed. The periblast, together 

 with tbe endoderm and the mesoblast, constitutes the so-called hypoblast ; 

 and for tbe cells left after the separation of the periblast, the author 

 suggests the term endoblast. The endoblast includes the gut-endublast 

 and tbe yolk-endoblast. From the former arise the embryonic meso- 

 blast and tbe endoderm of tbe gut ; from tbe latter the extra-embryonic 

 mesenchyme, the angioblast, or rudiment from which the blood-vessels 

 arise, and the yolk-sac epithelium. The author does not, therefore, re- 

 cognise mesoblast in the ordinary sense. He draws up the following 

 general statement of the parts played by the germ-layers. The epiblast 

 yields nerve-tissue, and the epidermal tissues; the hypoblast is differ- 

 entiated into (1) the embryonic mesoblast, which is the common rudi- 

 ment of the striped and unstriped muscular tissue, of the genital epi- 

 thelia, and of the embryonic connective-tissue ; (2) the extra-embryonic 

 mesenchyme ; (3) the angioblast, from which the blood and blood-vessels 

 arise ; (4) the endoderm, from which originates the epithelium or glands 

 of the gut. Finally, the lecithoblast, when it is present, also originates 

 from the hypoblast. The paper includes much more than can be indi- 

 cated in a brief summary, and is copiously illustrated. 



Fixation of Egg in Mouse.* — Dr. G. Burckhard has studied the 

 fixation of the egg to the uterine mucous membrane, and the formation 

 around it of the decidua in the white mouse, and discusses the general 

 bearing of his results. When the earliest stages only are considered, 

 two types of mammalian development can be distinguished. In the one 

 at the end of segmentation, a large embryonic vesicle is formed, on the 

 surface of which the rudiment of the embryo is placed, while the primi- 

 tive streak is also formed upon the surface of the embryonic vesicle. 

 The embryo does not become sunk below the surface until the formation 

 of the amnion. In the second type the embryonic vesicle is small, and 

 the primitive streak does not arise at tbe surface, but after the peculiar 

 fashion inappropriately termed inversion of the germinal layers, while 

 at a very early stage the egg becomes attached to the uterine wall by 

 means of its surface. In the first type, on the other hand, fixation 

 takes place relatively late. To the second group belong the mouse and 

 man ; and although the early stages have not been observed iu man, 

 there is reason to believe that they occur after much the same fashion 

 as in the mouse. Iu the mouse the small segmented egg begins to act 



o 



Arch. Mikr. Anat., Ivii. (1901) pp. 528-69 (3 pis. and 4 figs.). 



