504 DEVELOPMENT OF THE SIMPLE TISSUES, BY S. STRICKER. 



layers will only so far be considered as may be requisite to 

 render the histogenetic processes intelligible. 



An account of the unfecundated germ has already been 

 given in vol. ii., p. 192 et seq. To this exhaustive treatise I 

 have only to add a few observations in reference to the 

 nomenclature. I shall systematically avoid the use of the 

 expression "formative yolk" (Bildungsdotter) employed by 

 Reichert, and that of "principal yolk" (Hauptdotter) used by 

 His. Both expressions, as will presently be seen, are based 

 upon erroneous assumptions, and as neither can claim the 

 advantage of brevity, there is no good reason for discarding 

 in their favour the word " germ," used by Remak. It will, 

 in consequence of this, be further advantageous to term the 

 investing membrane of the germ (the zona pellucida of Baer), 

 not vitelline membrane (Dotterhaut), but blastoderm, germ- 

 tunic, or investment (Keimhulle). I shall only call this tunic 

 the vitelline tunic or vitelline membrane in those cases where 

 the germ exists together with a vitellus (food yolk, Reichert ; 

 secondary yolk, His) within one and the same membrane, 

 as in the eggs of Birds, scaly Amphibia, and osseous Fishes. 



It is generally admitted that the fecundated germ is at first 

 destitute of a nucleus.* This fact may best be demonstrated in 

 Batrachia, if a pair of animals in coitu be examined, whilst 

 some of the ova have been extruded, and others are still 

 retained in the body of the parent. It may then be shown, 

 either by tearing the fresh eggs in sunder, and examining 

 the contents as they escape with low powers, or by 

 section of the hardened ova, that each of the ova taken 

 from the body of the mother possesses a vesicular nucleus 

 (germinal vesicle), the membrane of which can, when fresh, 

 be divided with needles, with the aid of a lens ; in the 

 youngest fertilized ova, on the other hand, no nucleus can 

 be distinguished. This fact is interesting, since it shows 

 that the vertebrate animal begins as a non-nucleated mass. 

 If such non-nucleated Batrachian germs be hardened, there 



* Precise statements to the effect that the germinal vesicle is persistent, 

 and becomes transformed into the nucleus of the cleavage cells, has only 

 been made by Johann Miiller, in the case of the Entoconcha mirabilis. 

 Monateberichte der Berliner Akademie, September 1851. 



