138 Dr. 0. Zacharias on the Reproduction and 



vesicles), in opposition to other eggs, which always possess 

 only one nucleus ? " * 



By my observations a clear light is thrown upon the occur- 

 rence of a considerable number of nuclei in the eggs of many 

 Rotatoria, inasmuch as I have been able to ascertain that 

 between the blastomere-formation and the increase of the 

 original nucleus (by gemmation) there is an interval of several 

 hours. Accidentally I once met with an egg with four 

 daughter-nuclei, although no trace of the commencement of 

 segmentation was to be observed. Generally the grouping of 

 the vitellus into blastomeres commences when three nuclear 

 buds have separated. As we shall see by this the foundation 

 of the epiblast is already provided. By the formation of 

 grooves, the progress of which is scarcely perceptible, three 

 blastomeres are finally formed, each of which contains within 

 it a daughter-nucleus. These are seated like a saddle upon 

 the still unsegmented larger half of the egg, and appear then 

 to increase at the expense of the latter (which visibly becomes 

 smaller). The result of this increase is at the same time an 

 envelopment of the half of the egg which has remained un- 

 divided by the products of division of the original three blas- 

 tomeres — a process which leads to formation of a so-called 

 hood-gastrula. The outer lamella of this is represented by 

 the enveloping cell-mass, the inner one by the at first passive 

 half of the egg, which, however, begins to be segmented when 

 the envelopment has advanced so far that only a small aper- 

 ture (blastopore) still remains to be closed. 



When this closure has taken place the protoplasm of the 

 blastomeres, which have in the meanwhile increased in num- 

 ber and diminished in size by repeated division, becomes 

 fused together, and then is produced a tolerably dense layer, 

 furnished with numerous nuclei, which no longer allows us to 

 trace the fate of the great hypoplastic cell. But there can be 

 no doubt that morphologically this must be regarded as the 

 equivalent of an inner lamella, if we consider the details of the 

 process of segmentation in the egg of a wheel-animalcule 

 [Philodina roseola) nearly allied to Rotifer vulgaris. 



In the Riesengebirge, and especially in the neighbourhood 

 of Hirschberg (near the village of Grunau), we have a very 

 large species of the above-mentioned Philodina, the eggs of 

 which are quite peculiarly adapted for embryoiogical obser- 

 vations. The intestinal canal of this animal is of a cinnabar- 

 red colour, and we find the same pigment also in the vitelline 

 granules of the egg, a circumstance which is of particular 

 importance, as I remarked more and more as I continued my 

 * Zeitschr. f. wiss. Zool. vi. p. 102 (1855). 



