THE ANIMAL KINGDOM. 



version of tliis ball with its single layer of cells into a two-layered sac. We can 

 now compare the egg to a hollow rubber ball. By the pushing in of one side 

 (in the language of embryology, invagination) the ball can be made to resemble 

 the condition shown in Fig. II. H, the earliest stage of the gastrula. The single 

 layer of cells of the blastula is now differentiated into two layers, the outer of 

 which is varionsly termed epiblast, ectoderm or epiderm, the inner endoblast, hypoblast, 

 or endoderm'. So far, with the modifications to be noted below, these processes are 

 common to all animals above the Protozoa, and with the separation into two layers 

 we have the essential parts of a Hijdra ; the outer layer corresponds to the skin, the 

 inner to the digestive tract of that simple animal, as shown farther on in this volume 

 (p. 76.) In the higher animals — as, for instance, the frog — at one stage of the develop- 

 ment these two layers alone occur, and from the outer is developed the external layer 

 of the skin, while the hypoblast eventually forms the lining of the central ])ortion of 

 the digestive tract. We thus see that at this stage the embryo is composed of skin 

 and stomach, and the name gastrula means a little stomach. 



This epitome contains the gist of the earlier stages of a typical egg, but in nature 

 many variations occur, of which our space will allow but the merest mention. In eggs 

 which are composed of pure protoplasm this regular development occurs, but in most 

 eggs another element, known as food-yolk or deutoplasm, is 

 present, and accoixling to its distiibution the character of 

 the segmentation and of the invagination varies. The pro- 

 toplasm is the active joortion, the food-yolk, as the name im- 

 plies, is to nourisli the growing embryo, and its presence 

 tends to retard the development of the egg. Tlius, when 

 the jii'otojilasm is superficial, and the food-yolk occupies a 

 central position, as in the crustaceans, the segmentation 

 planes are at first confined to the surface, the central portion 

 remaining for a time vmsegmented. At other times the food- 

 yolk is confined to one pole of the egg, as in the frog, and 

 then the segmentation is, for a while at least, confined 

 to the protoplasmic pole, only affecting the other at 

 a comparatively late stage of development. This is 

 carried to its furthest extent in some of the fishes. All 

 of these modifications of the type of segmentation are 

 variously combined, so that great differences result, and 

 as in the eggs of the same class, or occasionally even of 

 the same genus, the distribution of the food-yolk will 

 vary, it will readily be seen that the segmentation is but 

 a very poor guide to the relationships of forms. In the 

 insects the segmentation is veiy greatly modified, but as 

 yet our knowledge is so slight as not to warrant any 

 broad generalizations on the earlier stages of the group. 



Correlated with the variations in the segmentation are certain modifications in the 

 gastrulation and the production of the epiblast and hypoblast. Let us i-eturn to the 

 normal' gastrula for a moment, and trace its progress just a little farther, naming some 

 of the points omitted above. The invagination is carried to such an extent that the 

 segmentation cavity is nearly obliterated, and at the same time the edges of the in- 

 folded ball are brought closer together, so that a comparatively narrow opening is the 



Fig. III. — Segmenting egg of 

 a Crustacean (Crangon). 



- Unequal segmentation 

 (fi-og). 



