Ill PLATODES— ONTOGENY OF POLYCLADA 167 



XIV. Ontogeny of the Polyelada. 



As a short illustration of the development of the Platodes from the fertilised egg, 

 we choose the Tiirlellaria {Polydadidm). [The ontogeny of the Ehabdocmla is 

 almost unknown, and the development of the Tridada seems to us to be markedly 

 ccenogenetio. The eggs of these latter animals develop at the expense of the numerous 

 yolk cells in the midst of which they lie imbedded within the egg cocoon, and it 

 might with justice be said that the eggs and embryos of the Tridada Uve parasitic- 

 ally on these yolk cells, which is not the case in the Folydada.] 



"We have abeady (Fig. 94, p. 125) described and illustrated the first stages of 

 segmentation. The 4 micromeres which are first separated by constriction yield the 

 whole ectoderm ; the next 4 or twice 4 in like manner produced form a large 

 part of the later mesoderm. The descendants of the 4 ectoderm micromeres grow 

 round the whole germ by repeatedly dividing, thus enclosing not only the 4 

 macromeres, but also the 4 or 8 mesoderm micromeres. They thus form at 

 last a continuous layer of epithelial cells round the whole germ, which is only broken 

 through at the vegetative pole by a longitudinal slit corresponding with the ventral 

 medium line of the embryo. This longitudinal slit is defined as the blastopore ; it 

 soon completely closes. The germ is now at the stage of a bilaterally symmetrical 

 planula, in which there is already a formation of mesoderm between the endodermal 

 rudiments (the 4 macromeres, which meantime by division of one of them have 

 increased in number to 5) and the ectoderm. 



The 4 or 8 mesoderm micromeres soon increase in number by fission, and 

 thus form either a ring of mesoderm cells or 4 masses of mesoderm cells (2 anterior 

 and 2 posterior). The macromeres, now surrounded on all sides, continue to give ofi' 

 by constriction micromeres, which again increase by fission and yield the intestinal 

 epithelium. The yolk-containing macromeres finally become disintegrated, and the 

 yolk is used up by the intestinal cells. Near the original blastopore a depression of 

 the ectoderm occurs, the stomodseum, as the first beginning of the pharyngeal 

 apparatus. The germ is now, apart from the fact that it is bilaterally symmetrical, 

 at the stage of a Scyphula or a young Ctenophoran larva. 



The mesoderm cells extend more and more between the endoderm and the ecto- 

 derm ; the sensory organs first appear in the ectoderm, on the side opposite the 

 stomodseum, near the original animal pole, but shifted somewhat along the median 

 plane, so that now anterior and posterior ends can be clearly distinguished. These 

 sensory organs take the form of 2 or 3 eyes, and of cells which carry tufts of long 

 hairs. In connection with these sensory organs, which in the case of eyes 

 soon sink down below the ectoderm and become mesodermal, the paired cerebral 

 rudiments arise as products of the ectoderm, and these also soon sink under the 

 surface and become mesodermal. The two rudiments become secondarily connected 

 by transverse bridges. The principal nerve trunks seem to arise as outgrowths of 

 the cerebral rudiments which form the so-called neural plate. 



The ectodermal body epithelium becomes provided with cilia at an early stage. 

 In the centre of the endoderm the enteric cavity arises in consequence of the 

 increasing absorption of the yolk by the endoderm cells, which arrange themselves 

 peripherally like an epithelium. Into the base of this cavity the stomadseum soon 

 breaks. The enteric aperture is thus formed. The stomodseum changes in the 

 following way into the definite pharyngeal apparatus. A circular invagination forms 

 in it, the first beginning of the pharyngeal pouch. This is surrounded by a collection 

 of mesoderm cells. Into the pharyngeal pouch the pharynx itself grows as a 

 circular fold, consisting of mesoderm cells and » covering of epithelium. The 



