CH. LIX.] 



SEGMENTATION 



883 



which the embryonic rudiments are converted into the more fully 

 developed condition in which they are found at birth is a subject 

 fully treated in works on anatomy, embryology, and obstetrics, and we 

 shall not go into those matters here. The nutrition of the embryo 

 and the circulation of its blood are, however, matters of physiological 

 moment, so that it will be necessary to refer to the origin of the 

 foetal membranes, as it is by their means that nutrition is carried on. 

 Let us, however, first briefly take up the early stages in development. 



SEGMENTATION 



After fertilisation is completed, the ovum divides into two parts ; 

 each of these again divides, and so on till a mulberry-shaped mass 

 the morula is formed. It consists of a large number of small cells 

 and it is enclosed together with the polar 

 bodies, in the zona pellucida. The polar 

 bodies soon disappear ; indeed in many cases 

 they have vanished long before the morula 

 is completed. A cavity soon appears in the 

 morula, which thus becomes converted into 

 a blastula or blastocyst. The cells which 

 form the peripheral wall of the blastula 

 assume a more or less cubical form, whilst 

 those which lie in the interior and form the 

 inner cell mass are irregular in outline, and 

 they are grouped together at one pole of the 

 blastula (see figs. 566, 567). At this period 

 the blastula is unilaminar, except at the 

 region where the inner cell mass is 

 situated; but soon the cells of the inner 

 mass extend round the cavity and the wall 

 of the cyst becomes bilaminar, the outer 

 layer being called epiblast and the inner 

 hypollast. A little later the mesoblast or 

 third layer of the blastoderm grows and 

 extends between the epiblast and hypoblast 

 over the whole area of the vesicle. 



That portion of the mesoblast which lies 

 immediately at the sides of the neural 

 groove becomes partially separated from the 

 rest, and at the same time divided into 

 cuboidal blocks, the protovertebrse or meso- 

 blastic somites (see also fig. 570). The more 

 laterally situated part of the mesoblast 



constitutes the lateral plates, and the narrow strand of meso- 

 blastic cells which connects the lateral plate on each side with the 



FIG. 565. Diagrams 



tin- 



early stages of cleavage of the 

 ovum. (Dal ton.) 



