90 PEOF. allma:n" on the 



splieres, and that, in accordance with this, the segmenting 

 ovum now j)i'esents spheres of unequal size. I showed that 

 successive segmentation goes on with great activity in some of 

 the spheres, while others continue unaffected by it, or present 

 it with much less energy, and that the former become broken up 

 into a multitude of cells, by which the latter become gradually 

 enveloped, so that the ovum now presents two A^ery distinct por- 

 tions — a central one, composed of large spherical cells, and a peri- 

 pheral one, of much smaller cells. 



It was shown that on the completion of the peripheral layer a 

 large lacunar cavity had become apparent among the cells in the 

 centre of the ovum, and that soon after this a depression (the 

 future mouth) takes place upon the surface of the embryo, and 

 becomes deeper and deeper until the depression formed reaches 

 the central lacuna, into which it finally opens. I overlooked, 

 however, a still earlier stage, in which, according to Chun, the 

 communication of the central cavity with the exterior takes place 

 by means of an orifice, which shows itself at the pole opposite to 

 that of the definitive mouth. To the embryo in this stage Chun 

 assigns the significance of a gastrula, with its two embryonic leaf- 

 lets, ectoderm and endoderm, its gastric cavity, and its mouth, 

 which, however, has but a temporary existence, closing after a 

 time, and being succeeded by the definitive mouth, which opens 

 on the opposite pole. 



I further showed that on the completion of the definitive mouth, 

 the ocellus, with its capsule and the rudiments of the oval tenta- 

 culiferous disks (pole-plates), make their appearance on the aboral 

 pole, and that the refractile corpuscles composing the ocellus may 

 at this time be seen to be developed in the interior of special cells. 

 The eight meridional rows of swimming-plates have by this time 

 made their appearance, not at equal distances from one another, 

 but in four pairs, extending from within a short distance of the 

 aboral pole along the sides of the embryo, over about one fourth 

 of the whole distance from pole to pole. 



The next stage observed was characterized by the development 

 of the vascular system. This commences by the difi"erentiation 

 of a portion of the large-celled tissue or endoderm, which consti- 

 tutes the greater part of the embryo, into two somewhat pyriform 

 masses, extending one on either side of the alimentary cavity, 

 and which, simultaneously with their formation, become excavated 

 by an extension of the central lacuna. They thus form two large 



