EMBRYOGENY 121 



Fourth stage: The gastrula {diploblastic, or two-layered, embryo). 



Fourth process: Appearance of a third layer. 

 Fifth stage: The triploblastic, or three-layered, embryo (not given a 

 particular name). 



Fifth process: Tissue formation. 



Sixth process: Organogeny (development of organs). 



It should be remembered that the stages are not stopping points, 

 that each of the processes lasts for a considerable time, and that the 

 whole forms a continuous development. As the cleavage cavity first 

 appears, the embryo is spoken of as an early blastula; as it increases in 

 size, an older blastula; and just before invagination begins, a late 

 blastula. In the same way reference may be made to an early and a late 

 gastrula. 



145. Variations in Embryogeny. — Since egg cells differ so much in 

 the amount and distribution of the yolk it will be clear that many varia- 

 tions in the course of embryogeny are bound to occur, and all the steps in 

 the ideal embryogeny described cannot be expected to appear in any 

 actual individual embryogeny. Different types of cleavage have been 

 previously noticed. In total cleavage the blastomeres may be spirally 

 instead of regularly arranged. It is then called spiral cleavage. When 

 the yolk is reduced to a minimum and the blastomeres are in contact 

 by only a small area, there may be no morula stage but cleavage maj^ 

 result in the immediate development of a gastrula (Fig. 48). When the 

 yolk is so abundant that the cleavage cavity is reduced to only a slit, 

 invagination becomes impossible, and the resulting overdevelopment 

 of the cells at the animal pole causes an outfolding. This is known as 

 gastrulation by epihole (Fig. 253(t). The archenteron is formed under 

 this fold, which may gradually grow around and envelop the whole 

 embryo. In the case of the mammal, as will be seen later, a very marked 

 change in the character of the embryonic stages results from the condition 

 which involves the attachment of the embryo to the wall of the maternal 

 uterus and its nourishment from the blood vessels of the mother. 



146. Germ Layers. — Reference has been made to three germ layers. 

 The blastoderm, appearing in the blastula, gives rise in the gastrula to 

 the ectoderm and entoderm, and the mesoderm is added in the triplo- 

 blastic embryo. These layers, in all Metazoa but the sponges, retain 

 this relative position, and from each arise a certain number of tissues. 



The tissues derived from the ectoderm include the epithelial covering 

 of the body, often known as the epidermis, which may extend inward a 

 short distance at the external opening or openings of the digestive cavity 

 or canal. They also include the epithelium lining all hollow organs the 

 cavities of which open to a surface covered by epidermis. This includes 

 such cavities as the external ear, the nasal chamber, and the spaces under 

 the eyelids. All nervous tissues are also derived from the ectoderm. 



