92 



INVERTEBEATA 



CHAP. 



stom 



Fig. 73. — Optical section of embryo of Berrie 

 fOTskalii showing the begiiniing of the eiido- 

 dermjil cavities. (After Clmn.) 



end, parencliynia-like endoderm ;, r, rib ; stom, stomo- 

 daeum whicli forms the so-called *' stomach." 



octants likewise throw oPf 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; 

 fnd 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 

 cells divide first and then 

 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 



micromeres rapidly 



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 ciha. Each streak corresponds to a pair of ribs. 



larger 

 extend 



Fxo. 74. — Optical section of embryo of Berije 

 forskalii in a later stage of development, 

 with a hollow endodermal sac. (After Chun.) 



Letters as before. In addition, ap, apical nervous 

 plate ; ot, otolithic concretions. 



