ORIGIN OF ORGANS. 69 
is still formed. The smaller, less yolk-laden cells, towards 
the animal pole, gradually grow round the Jarger yolk-con- 
taining cells, and a gastrula is formed by overgrowth or 
epibole. 
In various ways the ectoderm and the endoderm are 
established, either by some form of gastrulation, or by some 
other process, such as that called delamination (see p. 163). 
Mesoderm.—We are not yet able to make general state- 
ments of much value in regard to the origin of the middle 
germinal layer—the mesoderm or mesoblast. In Sponges 
and Ccelentera it is not a distinct layer except in Cteno- 
phora, being usually represented by a gelatinous material 
(mesoglea) which appears between ectoderm and endoderm, 
and into which cells wander from these two layers. In the 
other Metazoa, the middle layer may arise from a few 
primary mesoblasts or cells which appear at an early stage 
between the ectoderm and endoderm (eg. in the earth- 
worm’s development); or from numerous “ mesenchyme” 
immigrant cells, which are separated from the walls of the 
blastula or gastrula (e.g. in the development of Echino- 
derms); or as celom pouches—outgrowths from the en- 
dodermic lining of the gastrula cavity (eg. in Sagé?tta, 
Balanoglossus, Amphioxus); or by combinations of these 
and other modes of origin. The mesoderm lies or comes 
to lie between ectoderm and endoderm, and it lines the 
body cavity, one layer of mesoderm (parietal or somatic) 
clinging to the ectodermic external wall, the other (visceral 
or splanchnic) cleaving to the endodermic gut and its 
outgrowths. 
Origin of organs.—From the outer ectoderm and inner 
endoderm, those organs arise which are consonant with the 
position of these two layers, thus nervous system from the 
ectoderm, digestive gut from the endoderm. The middle 
layer, which begins to be developed in ‘“ Worms,” assumes 
some of the functions, e.g. contractility, which in Sponges 
and Ccelentera are possessed by ectoderm and endoderm, 
the only two layers distinctly represented in these classes. 
In a backboned animal the embryological origin of the 
organs is as follows :— 
(a) From the ectoderm or epiblast arise the epidermis 
and epidermic outgrowths, the nervous system, the 
