92 



CHAP. 



stem 



FIG. 73. Optical section of embryo of Jk-rfie 

 fm-skulii showing the beginning of the endo- 

 dermal cavities. (After Chun.) 



octants likewise throw off a smallest micromere, there arises in this 



way a crown of smallest 

 micromeres surrounding one 

 pole of the embryo. 



At this stage the embryo 

 consists of fifty - six cells ; 

 eight macromeres, and forty- 

 eight micromeres. The eight 

 macromeres now divide each 

 into two cells of equal size, 

 so as to form a circle of 

 sixteen large cells, and no 

 further divisions take place 

 in them until the micromeres 

 have completed their 

 multiplication. 



In subsequent divisions 

 of the micromeres, in the 

 outer octants, the smaller 



etui, parenchyma-like endoderm ; . r, rib ; stom, stomo- -,-, -,. ., ft f VI i-Vi 



daeum which forms the so-called "stomach." CBllfl QlVlde nrSt and 111611 



the larger micromeres divide. 



In the middle octants also the four smallest micromeres divide first, 

 whilst the eight larger micromeres flatten out and commence to spread 

 over the macromeres, after which they also divide. From this time 

 on, division of the micromeres 

 occurs rapidly, and the sheet of 

 cells to which they give rise 

 spreads more and more over 

 the surface of the macromeres. 

 The macromeres are originally 

 exposed both above and below 

 but the upper opening is nar- 

 rowed as the edge of the ring 

 of smallest micromeres extends 

 inwards, whilst the larger 

 micromeres rapidly extend 

 downwards over the surface of 

 the macromeres. This over- 

 spreading of the macromeres by 

 smaller cells is termed epibole. 

 It is a process often met with 

 in the development of animals 

 and is to be regarded as a variety 

 of gastrulation. 



There can now be seen, radiating from the upper pole, 

 four streaks of specially small and rapidly dividing cells ; these 

 are the forerunners of the " ribs " of the adult which carry the 

 comb-like plates of cilia. Each streak corresponds to a pair of ribs. 



stom 



FIG. 74. Optical section of embryo of Eeriie 

 forskalii in a later stage of development, 

 with a hollow eudodermal sac. (After Chun.) 



Letters as before. In addition, ap, apical nervous 

 plate ; ot, otolithic concretions. 



