INTRODUCTION 15 



through a cell Hneage the succession of blastomeres, and to 

 tell what organ or structure of the larva or adult is ultimately to 

 be formed from any given blastomere. This recognition of a 

 fixed arrangement of the cells is especially characteristic of anne- 

 lids and molluscs (Fig. 10). The annelidan cross and the mollus- 

 can cross are terms which are applied to the fixed cleavage 

 patterns characteristic of these respective groups. The orderly 

 processes which give rise to fixed relations of the cells of the 

 embryo are carried throughout the entire life of some organisms, 

 for scattered through the various phyla there are examples of 

 animals which in the adult stage possess an absolutely fixed 



a b c 



Fig. 10. — Diagrams showing cleavage pattern in MoUusca and Annelida, 

 a, blastomeres of Nereis iaftcr Wihon) ; h and c, blastomeres of Crepidula. 

 The stippled cells form the Molluscan Cross. The four unshaded cells at the 

 pole of the embryo are termed the apical cells, while the unshaded cells radiating 

 from these comprise the Annelidan Cross. {After Conklin). 



number of cells or of nuclei in all or in part of the organs of 

 the body. Such a condition is spoken of as cell constancy or 

 nuclear constancy. 



Blastula and Gastrula. — All eggs pass through a stage called 

 the blastula wherein the blastomeres are usually arranged in a 

 single layer. Typically, the cells surround a central fluid-filled 

 cavity, but in centrolecithal eggs the fluid-filled cavity is replaced 

 by a solid yolk mass. The cells in certain regions of the blastula 

 multiply more rapidly than the surrounding cells. Typically, in 

 the homolecithal egg this unequal growth causes part of the wall 

 of the blastula to be forced within the blastula cavity as an 

 inpocketing or invagination. This stage in development is 

 termed a gastrula and is composed typically of an outer layer of 

 cells, the ectoderm, and an inner layer, the entoderm, which 

 surrounds a central cavity, the gastrula cavity or archenteron. 

 Primitively, this archenteron is in communication with the exte- 



