122 whitman: [Vol.1. 



position was held to be sufficient evidence of derivation from 

 the three macromeres. The number of these cells is very soon 

 raised to six, but whether by renewed proliferation on the part 

 of the macromeres, or by subdivision among themselves, we 

 are not informed. A little later the still more numerous ento- 

 derm cells present the form of a solid axial cord, on each side 

 of which is seen a row of three mesoderm cells. These two 

 rows of cells are regarded as the basis of the germ-bands 

 ("Keimstreifen"), but the important question of their origin 

 is left undecided. The mesoderm cells increase rapidly, but 

 not by proliferation on the part of the vitelline macromeres, as 

 supposed by Kowalevsky. Biitschli expressly states that he 

 has never seen these macromeres in process of division, until 

 in a very late stage of the free embryonic life, when, as was 

 first shown by Robin, they break up into a number of cells, 

 which undergo resorption in the body-cavity (Bergh). 



In the next stage represented by Biitschli, a narrow, slit- 

 like lumen is seen between the entoderm cells, which is the 

 incipient enteric cavity. The number of entoderm cells must 

 be small, as only eigJit are shown in optical section (Fig. 9, 

 PI. XVIII.) ; and it must be noted as a still more remarkable 

 fact that precisely the same number of cells appear in a much 

 later stage (Fig. 12), after the formation of the oesophagus. 

 They have increased immensely in size, while the three macro- 

 meres have become proportionately smaller. Biitschli thinks 

 the entoderm cells enlarge at the expense of the fluid food- 

 material contained in the egg-case ; but his figures suggest that 

 the growth is at the expense of the macrorneres, and analogy 

 would lead one to suppose that the embryo would exhaust its 

 own stock of food-stuff before drawing upon external supplies. 

 It is evident that these large entoderm cells have many changes 

 to undergo before assuming the form of a lining epithelium; 

 but what these changes are, and how the cells multiply, are 

 questions left unanswered by Biitschli^s observations. I find 

 no mention of " free nuclei," and no indications of such bodies 

 in his figures, unless the two supernumerary nuclei, shown in 

 Fig. 12, may be so regarded. Such nuclei have been observed 

 in the egg of Nephelis, according to Balfour (No. 12, pp. 3-9). 



(12.) Balfour, F. M. A preliminary Account of the Development of the 

 Elasmobranch Fishes. Quart, your. Mic. Sc. XXII., p. 323. 1874. 



