GERMINAL LAYERS. 133 



This segmentation of the yolk seems to have been 

 first noticed by Swammerdam. The change in the 

 ovum is produced by the action of the sperm-cell or 

 male element upon the ovule with its nucleus or 

 germinal spot. The ovum then becomes fertilised and 

 undergoes remarkable changes in its constitution. 

 The two masses of protoplasm contained in the male 

 and female cells mingle, the nucleus or germ- spot dis- 

 appears, and shortly afterwards the above-mentioned 

 yolk-cleavage commences. The parent cell (cytula) 

 thus becomes of a mulberry-form (" morula " of 

 Haeckel), and then a kind of hollow is formed within 

 the circumference of the cells. 



This becomes dented inwards at one spot, and 

 somewhat elongated ; by which alteration that which 

 appeared as a single membrane becomes two through 

 the formation of a groove or a cavity with an outlet or 

 blastopore. This constitutes the bilaminar Blastoderm, 

 called also Grastrula by Haeckel because it always 

 comprises the alimentary organs. 



Thus the first stage of the development of the ovum 

 consists of an accumulation of food-material at one 

 part of the layer, whilst the cells become larger in this 

 part. An invagination or cupping in the wall of the 

 blastopore (Schaf er) occurs ; then the edges of this cup 

 approach, and the opening left forms what Haeckel* 

 considers to be the primitive mouth, or the " blasto- 

 pore " of Prof. Ray Lankester. This invagination, 

 which appears to obtain in all animals above the 

 Protozoa, would seem to occur at the spot where the 

 greatest amount of nourishing cells have accumulated. 



In this manner two germinal layers are formed; and 

 finally a third layer, which last, however, by some is 

 thought to have a less significance and to arise from 

 the first two. 



The body-cavity known as the Archenteronf is 

 probably never produced by invagination in the Tra- 



* BXaoroc, a germ ; nropog, a channel or pore. 

 t apxfi) commencement ; kvnpov, intestine.] 



