MAMMALIAN HOOD-GASTRULA. 215 



a crark brown colour when treated with osmic acid (Fig. 

 41, i). This dark central cellular mass is surrounded by 

 a brighter globular membrane, composed of 6-t smaller cube- 

 shaped and finely granulated exoderm-cells, which lie side 

 by side in a single layer, and take up very little colour from 

 osmic acid (Fig. 41, e). The exoderm-membrane is broken 

 only at one single point, when 1, 2, or 3 entoderm-cells 

 pierce to the surface. The latter form the yelk-plug 

 which entirely occupies the primitive mouth of the Gastrula 

 (o). The central primitive intestinal cavity is filled by 

 entoderm-cells (Plate II. Fig. 17). The single axis of the 

 outline of the mammalian Gastrula is thus clearly indi- 

 cated.^^ 



Although the unequal egg-cleavage and gastrulation of 

 Mammals and Amphibia present various peculiarities, it 

 is comparatively easy to trace these processes back to the 

 egg-cleavage and gastrulation of the lowest Vertebrate, the 

 Amphioxus, which is entirely similar to the form of cleav- 

 age carefully examined by us in the case of the Coral. (C£ 

 Fig. 22 and 28.) All these and many other classes of 

 animals agree in that, in their egg-cleavage, the whole eg^ 

 parts, by repeated division, into a large number of cells. 

 All such animal eggs have long been called holoblastic, a 

 name given them by Remak, because in them the cleavage 

 into .cells extends to the whole mass ; or, in other words, is 

 total (Plate II.). 



In very many other classes of- animals this is, however, 

 not the case ; for instance, among Vertebrates, in Birds, Rep- 

 tiles, and most Fishes ; among Articulated animals (Arthro- 

 foda), in Insects, most Spiders and Crabs; among Soft- 

 bodied animals (^Mollusca), in Cephalopods or Cuttle-fishes, 



