g K. MITSÜKURI; 



das ä assers te Ende desselben" (p. J'22). He promises to give 

 details in :inother paper. 



Will's views on the changes occurring in the part where the 

 cell-mass in question is situated are somewhat intricate, and the 

 reader must refer to the original papers (*92, '93, '95) for a 

 detailed description.* I can attempt to give here only the l)are out- 

 lines. 



The ])rimitive 

 plate+, situated 

 posteriorly to the 

 embryonic shield, 

 is at first in the 

 shape of a sickle 

 and the blasto- 

 pore found in the 

 middle of the 

 plate is a trans- 

 versely elongated 



pp, primitive plate; «sp, entrance to the archenteron ; sr, sickle yht rAVc^jodcut I 

 groove. 



A). Soon, how- 

 ever, the two horns of the sickle are grown over by the epiblast, by the 

 process of the epibolic invagination, and there remains only the middle 

 part of the primitive plate, which thus assumes a circular shape. By 

 rapid cell-multiplication, the primitive ])late now grows in length and 

 becomes a median longitudinal streak (the primitive streak) which 

 encroaches /o/'H'a/ïZ more and more on the embryonic shield (Woodcut 

 I. B). A section through the primitive plate shows that, at its edges, 



pp "r 



pp Î'SP 



Diagram of the surface views of two embryos of Cistiido lutarid 

 according to Will. (Zool. centralhl. I. p. 136) s, embryonic shield ; 



* Will himself has recently given a summary of his own work in Zool. Centralhlatt, I. Xos. 

 éjS, 8, Sf 9. For tlie point iu question, see pp. 136-139. 



t For the explanation of this term, see furtlier on, or Contrih. IV. 



