758 SUMMARY OF CURRENT RESEARCHES RELATING TO 



out the history of these bodies is to show that there is a regular replace- 

 ment of one generation of cells by another, and, owing to this, a second 

 may take the place of the one expelled. 



Early Changes of the Chick * — It is certainly to be desired, in 

 the nterests of embryological students, that the principal facts and 

 inferences relating to the chick's development could at length be 

 established beyond the reach of controversy. For, alike on historical, 

 practical, and scientific grounds, this accessible type seems likely to 

 maintain its place as the standard of comparison for the embryogeny 

 of the higher vertebrates, and especially for the study of their initial 

 phases. The rapidity, however, with which these phases succeed one 

 another, and the diverse changes which, within very short periods, 

 occur in different parts of the same organism, present difficulties of 

 observation so great that our most skilled experts have not yet been 

 able wholly to overcome them. Hence our best works, those of 

 Kolliker and Balfour, cannot be regarded as in all respects satisfactory. 

 Hoping to dispel some remaining doubts Dr. W. Wolff returns to this 

 familiar subject ; his observations suggest a discussion and review of 

 certain opinions of his predecessors. 



As regards cleavage there is not much to be said. Why, he asks, 

 should writers assert that it occurs more rapidly near the centre and 

 surface than in the deeper and outer parts of the germ ? JNo one 

 has proved this. A uniform rate for the change in question being 

 assumed, it must be over sooner in the region where it began. The 

 formation of the subgerminal cavity is best explained if we compare 

 it to the cleavage-cavity of mammals. Since the yolk retracts before 

 it divides, secreting at the same time a fluid mass, so, with the 

 approximation of the cleavage-spheres, becoming successively smaller 

 firmer and more numerous, their intercellular fluid likewise accumu- 

 lates and in the chick is lodged within the cavity formed by. the with- 

 drawal of the entire germ from the food-yolk below. No membrane 

 separates the nutrient yolk from the germ. The false appearance of 

 a membrane on the floor of the germinal cavity is produced artificially 

 by coagulation of a film of the fluid white yolk. 



The margin of the hitherto lenticular germ becomes, in the lower 

 portion of the oviduct, thicker than its centre, the central cells 

 probably spreading themselves as the primitive outer lamina is 

 developed. This " ectoblastoderma " is polyderic. Its cells are more 

 easily stained than those of the rest of the germ. The under surface 

 of the outer layer, in contact with this residuum, is uneven; but 

 whether, once formed, it receives cells therefrom or has henceforth an 

 altogether independent increase is not certain. There are now, in 

 the still unincubated germ, two kinds of cells and two only, (a) those 

 of the ectoderm, distinguished by their position, form, and chemical 

 characters, and (6) the cells beneath them, which do not constitute a 

 lamina and are best termed collectively the " remnant of the cleavage- 

 elements." These residual cells, compacted peripherally, are but 

 loosely grouped about the middle of the germ, which, partly on this 



* Arch. f. Mikr. Anat., xxi. (1882) pp. 45-64 (1 pi.). 



