ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 197 



which neural tube, chorda, and primitive segments are developed, so 

 that the cephalic and caudal elevations become joined by means of 

 a concave line. The ventral side of the trunk-region, on the con- 

 trary, is greatly swollen and bulges out ventrally and laterally like 

 a hernia, since it is filled with yolk-cells. This swelling is therefore 

 called the yolk-sac. 



In the further progress of development the embryo continually 

 -acquires a more fish-like shape. The anterior and the posterior 

 ends of the body, especially the latter, increase greatly in length, 

 and the middle of the trunk becomes thinner, for with the consump- 

 tion of the yolk-material the yolk-sac becomes smaller and finally 

 disappears altogether, its walls being incorporated into the ventral 

 wall of the intestine and that of the body. 



The interferences in the normal course of development become greater 

 in the same ratio as the yolk increases in amount, as it does in the 

 -case of the meroblastic eggs of Fishes, Reptiles, and Birds. With 

 the latter the yolk is no longer broken up into a mass of yolk-cells, 

 as in the case of the Amphibia ; it participates in the process of 

 cleavage, but only to a slight extent, inasmuch as nuclei make their 

 way into the layer of yolk which is adjacent to the germ, and, sur- 

 rounded by protoplasm, continue to increase in number by division. 

 The gastrula-form is altered until it becomes unrecognisable; only 

 a small part of its dorsal surface consists of cells, which are 

 arranged into the two primary germ-layers, whereas the whole 

 ventral side, where in the Amphibia the yolk-cells are found, is an 

 imsegmented yolk-mass. 



Thus we acquire in the case of the Vertebrates mentioned a 

 peculiar condition; the embryo, if we regard the yolk as not 

 ^belonging to the body, appears to be developed from layers that are 

 spread out flat instead of from a cup-like structure (Plate I., fig. 1, 

 page 213). Moreover we see even a greater distinction effected 

 between the dorsal and ventral surfaces of the egg during develop- 

 ment than was the case with the Amphibians. The fundaments of 

 all important organs, the nervous system, the chorda, the primitive 

 segments (Plate I., figs. 2, 8), are at first produced exclusively on the 

 former, whereas on the ventral side few and unimportant changes only 

 are to be observed. These consist principally in the extension of the 

 germ-layers, which spread out farther ventrally, grow over the yolk- 

 mass (Plate I., figs. 2-5), and form around it a closed sac consisting 

 of several layers. This circumcrescence of the unsegmented yolk by 

 the germ-layers is accomplished, on the whole, very slowly, the more 



