PRIMITIVE ANIMAL FORMS 115 



Indeed, this is practically what happens when hereditary or 

 other influences do not disturb the phenomenon. There are 

 organisms that remain practically thus throughout their 

 lives (Volvox, Protospongia, Magosphczra, etc.) ; and a fairly 

 large number of embryonic forms in the higher animals 

 momentarily adopt this shape, to which the name blastula 

 has been applied. As a rule this stage is soon passed. The 

 blastulae are usually covered with vibratile cilia, and it would, 

 indeed, be remarkable if the strength and activity of these cilia 

 were to act with strictly equal force on every side of the 

 blastula. If this were really the case the blastula would be 

 continually whirling around its centre. As a matter of fact, 

 there is always one region in which the cilia are more active, 

 and these draw the embryo in their own direction. Therefore, 

 the blastula has an anterior and a posterior extremity. It 

 elongates along its axis of locomotion and becomes ovoid. 

 The active anterior end is the region devoted to the con- 

 sumption of reserves contained within the constituent cells 

 of the blastula, and these reserves are accumulated in the 

 inactive posterior area ; thus the constituent substances of the 

 ovoid blastula are constantly attracted forwards. Hence, a 

 current is established which induces the posterior half of the 

 blastula to become invaginated into the anterior half. This 

 explains one of the more common developments in embryogeny. 

 The blastula thus becomes a gastrula ; its anterior 

 hemisphere, formed of transparent cells, remains external and 

 becomes the ectoderm : the posterior hemisphere, formed of 

 granular cells on account of the reserve substances it contains, 

 now becomes the interior la3 7 er and is known as the entoderm ; 

 the orifice, which is necessarily posterior owing to invagination, 

 is the blastopore. Various floating bodies, generally detached 

 from the entoderm, penetrate into the space separating the 

 ectoderm from the entoderm. These may fill the whole space, 

 or they may, in part, become attached to the inner, and in part 

 the outer side of the ectoderm, so as to leave a cavity between 

 them. This is the ccelom or general cavity, and the elements 

 between the ectoderm and the entoderm constitute the 

 mesoderm. The entoderm circumscribes the primitive digestive 

 cavity which, wherever a ccelom exists, is generally brought 

 into communication with the exterior by a second orifice 

 opposite the blastopore, which becomes the mouth. 



