Comparative Development. 31 



parent or germinating cell proceeds in such a manner as to carry the 

 layer of bright cells quite around the nutritive yelk enclosing it as in a 

 bladder, from which it is called the "bladder gastrula" or ( Hellenized ) 

 "perigastrula. '' The formation of the dark layer is effected later, and 

 inside of the bladder, beginning at one side where the bright membrane 

 suffers an indentation from which the dark cells spread themselves so 

 as to line the sphere, gradually consuming part of the }*elk in the pro- 

 cess. 



All the animals in the world that grow from an impregnated egg, pass 

 through one or another of these four forms of gastrulation. Some of 

 the protozoa, including the gregarinre and the monera amoeba, and other 

 rhizopods, do not pass through any type of gastrula, for the reason that 

 they do not spring from an egg at all. Many of them consist of only 

 a single cell, and when that cell is grown to maturity by the absorption 

 of nourishment, it undergoes one single cleavage, and no more, each 

 half becoming a complete animal with all the powers of the first cell. 



There are other animals of minute and simple organization, among 

 the flagellata and other infusoria, that develop to the form of a bell gas- 

 trula, but still, according to Huxley, may consist, of only a single cell. 

 And they continue their species, as all single cell animals must, by the 

 splitting of the cell into two. The bell animalcule is an example. 



There are also other minute and simple animals that rise above the 

 single cell and develop to some stage between the single cell and the 

 gastrula or cup form. Some of these stop at a development equal in 

 value to a morula or mulberry germ. 



There are many rhizopods that are in reality associations of simple 

 cells all alike, and each cell a separate organism, that is, the animal is 

 an association of single cell animals, each of an equal and homogenous 

 nature, the whole mass forming an animal called compound, but whose 

 functions and faculties are not superior to those of any cell composing 

 it. This sort of an animal is of the structural and functional value of a 

 germ morula. Hseekel names the " cystophrys, " described by Archer, 

 and the " microgromia socialis" and u labyrinthulae, described by Rich- 

 ard Hertwig, as special examples in point (see Evolution of Man, 2 

 58), still other mature and complete animals have substantially the form 

 and value of the blastula germ, a remarkable example of which is the 

 flimmer-ball or " magosphaera planula," as named by Haeckel. 



The gastrula stage of animal development, at the present day, is a 

 correct representation of a mature animal, which Haeckel names the 

 gastraea, which in very ancient times developed from protozoan or 

 amoeboid forms. 



When we rise into the animal subkingdom wl<iit< r<it<i, we find nu- 

 merous varieties whose mature development is only equal to a bell gas 



