PLATYELMINTHES[89].
Turbellaria.
Although there is perhaps no group in the animal kingdom the ontogeny of which would better repay a thorough investigation than the Turbellarians, yet the difficulties to be overcome have hitherto proved too great.
The fresh-water Rhabdocœla and Dendrocœla do not undergo any metamorphosis, and leave the ovum in a condition in which they cannot easily be distinguished in their general appearance from Infusoria. Many marine Dendrocœla also develop directly, while, as was first shewn by Joh. Müller, other marine Dendrocœla undergo a more or less complicated metamorphosis.
Marine Dendrocœla. Of the marine Dendrocœla which do not undergo a metamorphosis the form most fully worked out is Leptoplana tremellaris—(vide Keferstein, No. [187], and Hallez, No. [185]).
The ova are surrounded by large albuminous capsules secreted by a special gland. They are laid a great number at a time, and adhere together so as to form masses not unlike the spawn of nudibranchiate Molluscs.
Within the egg-capsule the ovum floats freely and undergoes a segmentation similar in many respects to the characteristic molluscan type. The ovum divides into two, and then into four parts, from each of which a small segment is then separated off. The four small segments, which appear to give rise to the epiblast, increase in number by division and gradually envelop the large segments[90]; so that an epibolic invagination clearly takes place. Between the small and the large cells is a small segmentation cavity, [fig. 86] A and B. At the time when twelve epiblast cells are present, each of the four large cells divides into two unequal parts (Hallez), [fig. 86] A. In this way four large (hy) and four small cells (m) are formed. The latter are placed at the opposite pole of the ovum to the epiblast cells, and give rise to the mesoblast, while the four large cells remain as the hypoblast.
Fig. 86. Sections through the ovum of Leptoplana tremellaris in three stages of development.
(After Hallez.)