214 NIKOLAS KLEINENBERG. 



above the lamina of clear cells, the disc assumes the form of 

 a cup, which by the narrowing of its mouth becomes gradu- 

 ally a typical blastodermic bladder, that is, a double-walled 

 sac, whose outer wall represents the ectoderm, while the 

 internal gives rise to the epithelium of the mid gut with its 

 glandular appendages. On comparing this with my descrip- 

 tion of the changes during the corresponding period of 

 development of L. trapezoides, considerable differences will 

 be noticed. I do not believe that essential errors can have 

 occurred on either side ; there must be a real difference in the 

 facts observed, and this is, in part at least, explained by the 

 peculiarities of the later development of L. trapezoides to be 

 now described. 



Immediately after its formation the gerrainative sphere 

 does not show a Avell-determined arrangement of the cells, 

 though there are differences in size between them. With 

 regard to the quality of the protoplasm it is the same in all 

 the cells. But after some time a grouping of the cells into 

 distinct layers begins, Avhich leads to the formation of the 

 germinal layers. The peripheral cells about one of the 

 poles multiply and become flatter, but it is to be noticed that 

 two of them — those situated at the most prominent point — 

 do not take part in this, but, on the contrary, increase in 

 size, and attain a considerable length ; these cells then 

 become covered by the small peripheral cells, and pushed 

 towards the centre (PL IX, fig. 3, cm.). In the inside, 

 upon these large cells, which I shall call mesoblastic, rests 

 a layer of small and flattened cells (en), and at their sides 

 are already distinguishable a small number of very thin 

 flatter-shaped cells [mes) ; these cells are closely united 

 together and arranged in two rows, which are directed from 

 the sides of the cells {cm) towards the opposite pole, where 

 they meet the remains of the embryoplastic material, consist- 

 ing of a layer of large and still undifferentiated cells. Thus, 

 the constitution of the laminse of the germinal layers is in 

 part marked out ; the flat peripheral cells (ec) form the 

 external layer (ectoderm), those collected in the interior 

 produce the internal layer (endoderm), and the few cells 

 grouped in two lateral columns {mes) are the first rudiment 

 of the middle layer(mesoderm) j all the large cells occupy- 

 ing the other hemisphere undergo further changes, tending 

 to produce an arrangement completely corresponding with 

 that just described. But before this occurs a division of the 

 germ into two hemispheres always becomes evident. While 

 the egg is elongating in one diameter a transverse furrow 

 appears half way between the two extremities ; it does not, 



