THE CTENOPHORA 



have been confirmed in all essential particulars by the unpublished 

 researches of Mr. T. H. Riches. The segmentation is holoblastic. 

 By three successive meridional cleavages the ovum is divided into 

 eight blastomeres, in each of which the granular protoplasm is 

 aggregated at one pole, the vacuolar deutoplasm at the other 

 pole (Fig. IV. 2). By an equatorial division a portion of the 

 granular protoplasm is next segmented off from the upper pole 

 of each blastomere, the embryo now consisting of eight upper 

 protoplasmic micromeres and eight large inferior macromeres (3). 

 The succeeding divisions lead to increase of the number of micro- 

 meres which are formed partly by continued budding off of small 

 cells from the four macromeres, partly by division of the eight 

 micromeres first formed. When some thirty to fifty micromeres 

 are present the macromeres cease to bud off fresh micromeres and 

 themselves divide. Reference to Fig. IV. 4 shows that the eight 

 macromeres are not all of equal size. There are four larger macro- 

 meres, median and inferior, and four smaller macromeres, lateral 

 and superior. The median macromeres divide first, the lateral 

 somewhat later, and this sequence is followed through the suc- 

 ceeding steps of development. In the next stage (6) the embryo 

 is ring-shaped, consisting of a circlet of sixteen macromeres 

 surrounding a central cavity widely open both above and below. 

 On one aspect, which we may at once call the aboral aspect, the 

 macromeres are covered over by the continually increasing cap 

 of micromeres. The micromeres at this stage show a four-rayed 

 symmetry, and on the aboral aspect they surround a cross-shaped 

 opening, the pseudoblastopore, erroneously described by Chun (6) 

 as the blastopore. The micromeres spread more and more over 

 the surface of the macromeres and extend towards the lower 

 surface. The next stage leads to the formation of the mesoblast. 

 The nuclei of the sixteen macromeres, which at first were situated 

 near the aboral pole, travel towards the opposite pole (7). The 

 micromeres meanwhile have increased in number, the size of the 

 pseudoblastopore is decreased, and there is at the lower pole a 

 roughly quadrilateral area bounded by micromeres which is the 

 true blastopore. Next follows a fresh division of the macromeres ; 

 first the eight median, later the eight lateral macromeres bud off 

 each a small cell at the blastoporic pole, thus there is formed a 

 median group of sixteen cells, which are the mesoblast. The 

 three germ layers are now established. The micromeres form 

 the epiblast, the macromeres the hypoblast, and the sixteen cells 

 above mentioned are the mesoblast. Thus far the embryo has 

 been formed by epibolic growth of the epiblast over the hypo- 

 blast. This is now succeeded by a process of embole. The 

 macromeres are rotated in such a manner that their previously 

 lower ends face inwards, their previously upper ends face out- 



