DEVL:i-or.Mi;xr OK i'l.vnui.a ix a im.i .milaijiax iivkijoii). *I7 



sogmentiition-cavity only on the lowor half of the embryo. 

 In the earliest stages of development the inner layer would 

 nppear to arise rather hy diffei-ent iatioii tlian by delaniination. 

 TIic outer cells or ectoderm {ecf.^) form a, layer one cell thick 

 above and several cells thick below. The cells of the placental 

 tissue in the space between the cup of the endoderm and the 

 lower two-thirds of the embryo also show differentiation. 

 Some of the cells are rounded or somewhat stellate ; they are 

 further distinguishable by staining much more readily, and 

 they appear to consist of protoplasm denser than that of the 

 other cells. It is possible that these cells are the more active 

 agents in transferring nutriment to the groAving endjryo. 



The end)ry() continues to grow, and the cells of whicli it is 

 composed increase greatly in number, but their size remains 

 the same as before. The embryo becomes pear-shaped 

 (fig. 15). The inner layer of flat endoderm cells {t'll(L'^) is 

 beginning to creep upwards, but the upper half of the 

 embryo is still only one cell thick and consists of the outer 

 ectoderm layer. The embryo as a whole is sinking down- 

 wards, and the endodermal cup formed from blastostyle and 

 gonophore is becoming deeply flask-shaped. The effective 

 placental tissue {2^1. t.) now lies only between the embryo and 

 the endoderra-cup. The outer portion of this tissue (e. E.), is 

 extending downwards and consists of extremely elongated 

 cells. At the mouth of the flask the ectoderm layer has 

 separated slightly from the perisarc and forms a definite 

 epitlielium adhering to the endoderm, and leaves a slight 

 crack-like space {d. S.), extending from the pei'isarc to the 

 pointed apex of the embryo. 



In the next stage the embryo has sunk still further towards 

 the centre of the gonangium (fig. 10). The ectoderm of the 

 embryo has become moi*e than one cell thick all round, and 

 the endoderm also is several cells thick below and has com- 

 pletely surrounded the blastococl (J5/.). The placental tissue 

 (j^l. f-) is well-developed, while the outer portion {c. E.) 

 retains its trausparent, attenuated condition. The flask- 

 shaped endoderm layer {g. e.) derived from the blastostyle 



VOL. 3, PART 1. 



