RESEARCHES ON THE DICYEMIEiE. 137 



whilst to the others he has given the name of B.hon)bogenous 

 (,oo/.tj3oc, a top). The Nematogenous Dicyetnidoe are gene- 

 rally larger; the cell of the endoderm terminates in a point 

 in the cephalic dilatation ; the number of cells of the ectoderm 

 forming the trunk is often greater, and finally the contents 

 of the axial cell are very different (fig. 3). 



The germs producing vermiform young measure 0012 to 

 0014 mm., and have a small spherical nucleus with a 

 pointed nucleolus ; the protoplasm is but slightly granular 

 and is not stained by picrocarmine, though the nucleus be- 

 comes rose coloured under the action of this reagent. The 

 germs are formed endogenously, in the network of the axial 

 cell,often far from any pre-existing germ ; they appear as small 

 spherical bodies with a well-defined boundary line, and are at 

 first homogeneous, with a small pointed globule in their 

 centre. The contour line thickens, and presently a thinner band 

 of a more pronounced tint, which thickens and eventually 

 becomes the body of the germ-cell, is seen encircling the 

 nucleus. This description does not quite agree with that 

 given by Strasburger as to the free formation of cells in the 

 vegetable kingdom, but the differences are perhaps due to 

 the very different external circumstances under which de- 

 velopment occurs. When a germ has become mature it 

 divides into two identical cells, each of a semi-spherical 

 shape, and each having a nucleus and nucleolus. These cells 

 divide again into four, which are identical in appearance 

 (fig. 6), though not so in reality, as is seen by following the 

 course of their development, for they do not divide altogether, 

 but three break up whilst the fourth remains intact. The 

 embryo in the next phase is composed of seven cells, of Avhich 

 six are similar whilst the seventh is much larger (fig. 7). 

 The six cells divide afresh, so that the embryo now consists 

 of thirteen cells, which form a kind of cap to the large cell, 

 which moulds itself to the cavity so formed ; at this stage the 

 cap has the form of an ovoid, truncated at its two extre- 

 mities, the central cell closes the orifice in this ovoid, 

 so that the embryo at this stage of development is an epi- 

 bolic gastrula, for the solution of continuity in the layer of 

 the ectoderm is the blastopore. This gastrula resembles that 

 of an osseous fish, and the Dicyemidse remain during their 

 whole life in this transitory phase of the evolution of fish. 

 After the gastrula has formed, the blastopore closes, the 

 central cell elongates and becomes spindle shaped, whilst 

 the embryo increases in size ; up to this point all the cells of 

 the ectoderm are alike, being cubical and transparent, and 

 each having a spherical nucleus with a nucleolus. The pro- 



