GASTRULA OF THE MAMMAL. 275 



which likewise coalesce at the edges. Thtis two main tubes are formed, one 

 above the other. During the formation of these, the embryo separates into 

 strata, so that the two main tubes are composed of subordinate tubes which 

 enclose each other as fundamental organs, and are capable of developing 

 into all the organs." — Kakl Eenst Baer (1828). 



The mammalian egg, in the stage of development in which 

 we left it, presented an extremely important and remark- 

 able germ-form, the Gastrula (Fig. 41, p. 213, and Plate II. 

 Fig 17). The whole body of this globular Gastrula con- 

 sists solely of the two kinds of cells which compose the 

 two primary germ-layers. A single stratum of lighter- 

 coloured and firmer cells forms the outer germ-layer, and con- 

 stitutes an outer covering over the whole surface of the body 

 of the Gastrula. The whole interior of the latter is filled 

 by the darker and softer cells of the inner germ-layer : it 

 is only at a single point that these latter cells appear at 

 the outer surface of the spherical body; this point is the 

 mouth of the Gastrula, the primitive mouth (jprotostoma, 

 Fig. 41, 0). 



It is no easy task to explain how the complex mamma- 

 lian organism originates from this simple Gastrula. In 

 order to lighten the task, we have, as a preliminary, made 

 ourselves acquainted with the typical structure of the 

 simple primitive Vertebrate (Fig. 52-56, p. 256). We chiefly 

 based our study of that directly on the real conditions 

 which may yet be actually seen in the structure of the 

 body of the lowest extant Vertebrate, the Amphioxus. In 

 most important points of internal organization we may 

 regard the Amphioxus as a correct, palingenetic picture of 

 the long-extinct parent-form of all Vertebrates, the form to 

 which the origin of Man must also be referred. It is only 

 in a few unimportant points that the Amphioxus appears to 



