EXPLANATION OF PLATE XXII. 



Upper Jour figures. Stages in the development of the egg of Sagitta. 

 Slightly modified from ELPATIEWSKY (1909). 



a. Conjugation of egg and sperm nuclei. Polar nuclei below, germ-cell 

 determiner (" besonderer Ko'rper ") above. 



b. Four-cell stage germ-cell determiner in one blastomere. 



c. 32-cell stage. The cell of the i6-cell stage which contained the germ- 

 cell determiner has divided into tw.o cells, an upper which forms the 

 primitive germ-cell and a lower which is the first primitive endoderm-cell. 



d. Section of gastrula with two primitive germ-cells in archenteron. 



Lower five figures. Stages in the fertilisation and segmentation of Cyclops, 

 after AMMA. 



a. Sperm and egg nuclei in contact. Beginning of first cleavage spindle and 

 first appearance of ectosomes. 



b. Metaphase of first cleavage ; ectosomes at upper pole. 



c. Anaphase of second cleavage ; ectosomes at upper pole of one cell. The 

 larger granules are ectosomes persisting from the preceding division. 



d. i5-cell stage, 4th division of the granule-containing cell. Ectosomes at 

 the lower pole. 



e. Completion of 4th division of granule-containing cell into a lower cell, 

 G, with granules, which forms the primitive germ-cell, and an upper cell, 

 E, which gives rise to the endoderm. The other blastomeres are under- 

 going their fifth division. 



