652 I^- R- Cary, 



a compact mass, in which all outlines of the separate chromosomes 

 are soon lost. 



The constriction of the nuclear membrane continues until the 

 daughter nuclei are separated as though by amitosis. 



8. The formation of the spireme for the maturation division 

 takes place without the concentration of the whole of the nuclear 

 contents into an enlarged caryosome. Instead, the caryosome be- 

 comes directly converted into the spireme thread without the inter- 

 vention of a stage in which the chromatin exists is small granules. 



Usually the maturation spindle is excentric in the egg; it is 

 located near that end of the nucleus which lies nearest to the 

 cell w^all. 



In the formation of the polar body a portion of the nuclear 

 membrane containing one set of the daughter chromosomes from the 

 maturation division becomes extruded from the cell so that the polar 

 body contains no cytoplasm, but consists of a portion of the nucleus 

 surrounded by a double membrane. 



9. The mitosis in this form resembles closely that occuring in 

 the protozoa. 



10. The two micromeres, which represent the ectoderm, are 

 given off from the single macromere (mes-entoderm), in the first two 

 divisions. The gastrulation is epibolic. Two of the ectoderm cells, 

 in the nine cell stage, are set apart from the rest to form the in- 

 vesting membrane. The embryo soon becomes a syncytium, but the 

 separation between the ectoderm and endoderm usually remains 

 distinct for some time after the cell walls are lost. Sometimes 

 another la3^er of cells (nuclei and cytoplasm) may be distinguished 

 between the ectoderm and endoderm. 



11. The primordium of the gut often retains its identity from 

 the stage when it exists as a single cell after the separation of the 

 two primary micromeres. The i)rimorclium of the anterior sucking 

 disc, pharynx and oesophagus becomes marked out at the anterior 

 end of the embryo. The water vascular system arises as two lateral 

 intracellular tubes, one on either side of the body. The flame cells 

 arise in connection with some of the meristem nuclei and later ac- 

 quire their connection with the lateral trunks. 



The reproductive organs arise first as a single mass of cells, 

 which later becomes separated into recognisable primordia of 

 the testes, the ovary and the accessory parts of the reproductive 

 system. 



