412 MificelhoiPons. 



defined furrow between them, while the two lateral cells are sepa- 

 rated from one another by the anterior and posterior ones. ThuB 

 the blastula already exhibits a bilaterally symmetrical structure. 



The hindcrmost macromcre may be termed the endo-mesoderm 

 cell, since it divides into two cells, of which the one retains the 

 position of the posterior macromere, while the other, movinj? in the 

 longitudinal axis, passes more towards the animal pole. This cell 

 then similarly divides into two, but in the direction of the longitu- 

 dinal axis, and the two cells thus ])roduced are the primitive meso- 

 derm cells, which lie next one another on both sides of the longitu- 

 dinal axis, dorsally to the posterior macromere. 



After these processes have taken place the three other macromeres 

 divide, but simultaneously with the fourth, which had superseded 

 the endo-mesoderm cell, and furnish the endoderm cells of the wall 

 of the arehenteron. In the meantime the blastula Hattens out 

 dorso-ventrally, since the endoderm cells which have arisen from 

 the macromeres, as well as the two primitive mesoderm cells, are 

 surrounded by the ectoderm and pressed into the segmentation- 

 cavity. With progressive flattening of the germ the arehenteron is 

 graduall}' formed by invagination of the endoderm, its walls being 

 constituted by the progeny of the four macromeres. The flattening 

 finally becomes so great that the segmentation-cavity is reduced to 

 a cleft, while the blastopore forms an elongated oval which is situ- 

 ated in the longitudinal axis. 



During this time the two primitive mesoderm cells, which on the 

 invagination of the endoderm had eomc to lie at the hinder pole in 

 the segmentation-cavity, have given rise to a mosodermic band on 

 either side of the arehenteron. Simultaneously with this the 

 embryo has also lost its rounded form, and when seen from the 

 ventral or dorsal surface appears as a spherical triangle with nearly 

 equal transverse and longitudinal axes, and with the apex directed 

 forwards and rounded angles. 



At the next stage the blastopore forms a long slit, which occupies 

 the whole length of the ventral side. The communication between 

 the arehenteron, which possesses a tolerably wide cavity, and the 

 exterior persists at about the middle of the bla-^topore. while the 

 edges of the blastopore elsewhere grow together. The mouth 

 proceeds directly from the persisting communication between the 

 blastopore and the exterior. The first traces of the velum now also 

 appear in the shape of a double row of clear ciliated ectoderm cells, 

 which form a girdle directed oblicpu^lv to the longitudinal axis, and 

 whicli bisects the longitudinal axis in the dorsal median line and in 

 the ventral median line passes in front of the ;uiterior end of the 

 blastopore. The mesoderm has become bilamellar and forms a 

 saccule on each side on the right and left, which pass into one 

 another at the hinder i)ole and gradually grow out forwards and 

 dorsally. The coelom lying between the two layers of the meso- 

 derm is distinctly visible. 



The arehenteron soon changes its shape. It is broader in front, 

 with a wider lumen, and narrows towards the hinder end, whereby 

 its lumen becomes corresporulingly smaller. In lateral view it is 



