NOTES, 471 



are given with these " Communication preliminaire ; " Van 

 Beneden's description is, however, so clear, so thorough and care- 

 ful, that they afford an entirely satisfactory insight into unequal 

 egg-cleavage and the formation of the Hood-gastrula in Mammals. 

 All other observers, who have studied the germination of Mam- 

 malian eggs (among the most recent Kolliker, Rauber, and 

 Hewson may be especially mentioned), have overlooked or 

 failed to recognize the important features discovered by Van 

 Beneden. 



70 (i. 218). The Disc-gastrula (Disco-gastrula) of Osseous 

 Fishes (Tdeostei). Van Bambeke, " Recherches sur 1'embry- 

 ologie des poissons osseux." Brussels, 1875. The transparent 

 Fish-eggs, in which I observed discoid cleavage (Segmentatio 

 discoidalis) and the formation of the Disc-gastrula by invagination, 

 are accurately described in my article on " The Gastrula and 

 Egg-cleavage of Animals " (" Jen. Zeitschrift fur Naturwis- 

 senschaft," 1875, vol. ix. p. 432-444; Plates IV., V.). On the 

 Disc-gastrula of Selachii, cf. Balfour, " The Development of 

 Elasmo branch Fishes." " Journ. of Anat. and Physiol.," vol. x. 

 p. 51 7; Plates XX, XXIII. 



71 (i. 221). Yelk-cells of Birds. The cell-like constituent 

 parts, which occur in great number and variety in the nutritive 

 yelk of Birds and Reptiles, as in most Fishes, are nothing less 

 than true cells, as His and others have asserted. This does not 

 mean that in this matter a distinct limit everywhere exists 

 between the nutritive and the formative yelks, as in our oceanic 

 Fish-eggs (Figs. 42, 43, note 70). On the contrary, originally 

 (phylogenetically) the nutritive yelk originated from part of the 

 entoderm. 



72 (i. 223). Egg-cells of Birds. Notwithstanding the large 

 nutritive yelk, the " after-egg " (metovum) of Birds and Reptiles 

 is, in form- value, a single cell. The very small, active protoplasm 

 of the " tread " does, however, indeed fall far short, in volume, 

 of the huge mass of the yellow yelk-ball. The bird's eggs are 

 absolutely the largest cells of the animal body. Cf. note 43, and 

 Eduard van Beneden, " Recherches sur la composition et la 



