6 ZOOLOGICAL EXERCISES. 
an animal, this portion of Morphology is distinguished under 
the special name of Development. 
In the lowest animals—the Protozoa—the body is formed 
of one cell only, which develops directly from the germ > 
but in all the higher animals — called collectively the 
Metazoa—the original cell, called in this case an ovum, 
divides into two, four, eight, &c., parts, which remain con- 
nected, and gradually build up the individual. The ovum 
consists of a mass of granular protoplasm called the vitellus,. 
or yolk, containing in it a clear space, called the germinal 
vesicle, in which is seen a small spot, the germinal spot. 
It is the germinal vesicle that undergoes division, and the 
division may spread through the whole yolk, or it may be 
confined to a portion of it. The cells which result from the 
division, or segmentation, of the germinal vesicle, are called 
blastomeres, and the tissue to which they give rise the 
blastoderm. The blastoderm differentiates into two layers 
of cells, the outer of which is called the epiblast, the inney 
the hypoblast. 
When the segmentation of the yolk is total, the blasto- 
meres form a mass of cells called a morula. The inner 
blastomeres liquify, leaving the blastoderm surrounding a 
hollow cavity. This is called a planula. <A portion of the 
blastoderm now gets pushed in, so that the hypoblast gets. 
enclosed within the epiblast. This is done either by invagi- 
nation or emboly, like pushing in with the finger a portion 
ot a hollow india-rubber ball, or by the cells of the epiblast 
increasing more rapidly than those of the hypoblast, and so 
growing over them; this is called epiboly*. The embryo 
now 1s called a gastrula. When the seementation is partial, 
the blastoderm is small in comparison to the yolk, and grows 
_ * It would appear probable that emboly is only « modification of 
epiboly, 
