i6 



PROBLEMS OF EVOLUTION 



is repeated again and another polar body passes, out. The 

 egg is now ready for fertilisation, and, when this has taken 

 place, development goes rapidly forward. It first divides 



FIG. 4 A. Pandorina Morutn A, entire colony ; B, asexual reproduction, 

 each zooid dividing into a daughter colony. (From T. J. Parker after 

 Goebel.) 



into two similar cells, then each of these again divides. When 

 a number of cells are thus formed it is called a morula from 

 its resemblance to a mulberry (fig. 3) a stage found in the 

 ontogeny of the sponge and all animals above 

 it in rank. 



In this stage it is very similar to 

 Pandorina, a creature found in pond-water. 

 In the illustration Pandorina is seen to 

 consist of a colony of sixteen one-celled 

 animals enclosed in a thin membrane. In 

 B each of the sixteen is dividing into 

 sixteen. When the envelope breaks there 

 will be sixteen daughter colonies. The 

 FIG. s.-Gastruia in longi- next important stage is the gastrula where 



j?T.SEi: the P rimitive <% estive ca y i s -en. 

 d, ectoderm. (From For sea anemones and fresh water hydras 



Parker and Haswell. .1- . J 



after Gegenbaur.) tms 1S not a passing phase, but the final 



goal of their development. They are bags 



with double walls, gastrulas in fact, with a number of 



