380 
REPRODUCTION — MORULA AND GASTRULA. 
'Die eggs of our British species contain comparatively little stored- 
up food-yolk and the segmentation is therefore complete or Holohlastic 
(o/\os, whole; BXaa-Tij, bud) and only exceptionally incomplete or 
j\Ieroblastic part ; /SXaa-Ti], bud) as in the Cephalopoda, in which 
a large part of the egg is formed of nutritive substances and takes no 
part in the division, which is always most active and complete at the 
animal or formative pole. 
The segmentation cellules, however, vary in size and are distinguished 
as Micromeres and Macromeres, the cleavage process being known as 
E(pial Segmentation when the cellules are approximately ecpial in size, 
and Uiieiiual Segmentation when they are markedly different. 
The Micromeres or segmentation cellules of the animal pole are 
small and poor in nutritive yolk, they originate at the point of expul- 
sion of the polar bodies, and give rise to 
the ectoderm, the mantle, the embiyonal 
and adult organs of locomotion, the nervous 
and sensorial .systems. 
The jMacromeres, or nutritive cellules, are 
larger than the Micromeres, and richer in 
yolk, they divide more slowly and occupy 
the opposite pole, and constitute the Eiidoderm, retaining the primi- 
tive functions of receiving and digesting food. 
The iMoRULA (dim. of morum, a mulberry). Polyblast or Blastula, 
constitutes the first .stage of develop- 
ment, the segmentation having been 
continued until the vitellus has ex- 
ternally the aspect of a uniform 
mulberry-like mass of nucleate cells, 
the interior forming the segmentation f IG. 715. — Morula stage of Pisidiuin 
. . pusilliDH (Gmelin), highly magnified 
cavity 01' Jilastococl. (after Ray Lankester). 
The Gastrula (dim. of yaa-ryp, the stomach) forms the second 
phase of development, and has the form of a sac, its ectoderm or outer 
layer being formed by the micromeres, and the inner layer hy the 
macromeres or endoderm cells, the gastrula arising in those eggs with 
little food yolk by the emboly or invagination of the macromeres or 
vegetative cells within the more rapidly multiplying micromeres or 
formative cells, but where more food-yolk is present the gastrula is 
formed by epiboly or overgrowth of the macromeres by the micro- 
meres, as in Blpkarium, etc.; the invagination is known as the 
F IG. 711. — Egg of lUthynia at 
formative pole, showing the un- 
equal segmentalionof the vitellus 
into macromtres and micromeres 
(after Pelseneer). 
