16 INVERTEBRATE ZOOLOGY 



rior through an opening, the blastopore. A gastrula such as that 

 just described occurs only in the homolecithal type of egg. In 

 telolecithal eggs, with total cleavage, the small blastomeres at the 

 animal pole multiply much more rapidly than do the heavily yolk- 

 laden vegetative cells. As a consequence, the cells from the 

 animal pole grow down and surround those of the vegetative pole 

 which thereby become the entoderm, while the small surface cells 

 are recognizable as ectoderm. In such a gastrula, there is no 

 archenteron within the entoderm. Gastrulation by this method 

 is termed epibolic gastrulation. In telolecithal eggs with only 

 partial cleavage, a modified invagination occurs through a shoving 

 in of cells near the edge of the cleavage disc. As mentioned 

 above, blastula formation in the centrolecithal eggs gives rise to a 

 layer of cells arranged around the central yolk mass. In later 

 development, each cell of this blastula undergoes a cleavage 

 parallel to the surface of the egg. Thereby a two-layered condi- 

 tion or a gastrula is attained and the process is spoken of as 

 delamination. 



Mesoderm and Later Development. — The addition of the third 

 body layer, the mesoderm, between the ectoderm and the ento- 

 derm is one of the most conspicuous and most significant features 

 in later development of all animals above the coelenterates. 

 Development from this point onward is subject to so many indi- 

 vidual differences that few general statements may be made. 

 In many of the more generalized groups, the attainment of the 

 gastrula stage marks the beginning of an independent existence. 

 By means of cilia the larva has the power of movement and 

 through the blastopore food is taken into the archenteron where it 

 is digested and assimilated. However, in many groups and espe- 

 cially in terrestrial and fresh-water forms, the individual is carried 

 far past the gastrula stage while still confined within the egg 

 membranes. In the extreme of such cases, the individual 

 which emerges from the egg is essentially like the adult except in 

 size and stage of development of the reproductive organs. 

 Throughout its development, the young of such a form would be 

 referred to as an embryo. On the other hand, if the individual 

 produced from the egg lacks some structures characteristic of the 

 adult and possesses others which are lost in later development, the 

 young is ordinarily termed a larva. Many different larval forms 

 are encountered in the various invertebrate groups but these are 

 so numerous and have such diverse forms that descriptions of 



