Dr. W. Salensky on IliickeVa Gastraea Theory. 17 



many instances occur at a much later period, after the com- 

 pletion of the segmentation. In most such cases the segmen- 

 tation-cells arc regularly evolved ; there arc 2, 4, 8, 10, &c. 

 cells, which further divide with the same regularity; in a word, 

 a regular segmentation takes place, producing a solid sphere 

 consisting of uniform cells. For this stage we may retain the 

 name of " Morula" by which Htickel indicates the so-called 

 mulberrv'-stage of segmentation. The Morula may become 

 differentiated in different ways. It may at once form the 

 embrs'o itself, becoming covered with a cuticular membrane 

 and cilia, and escaping as a larva — as is the case, for example, 

 in the digeneous Trematoda* {Amphistomum subdavatum &c.), 

 according to the observations of E. van Beneden. The larva 

 of these animals consists of uniform cells and is covered ex- 

 ternally Avith a ciliated membrane. The lar^^a of the Trematoda 

 can proljably Ijecome fuither differentiated and even acquire a 

 stomachal cavity. 



The Cestoda pass through the J/orw^-stage in the e^g. 

 Before the embryo escapes from the Qg(^, the differentiation of 

 its cells commences in it. This differentiation differs in its 

 results from the differentiation of the germ-lamellae, although 

 the processes are the same in both cases. In consequence of 

 the differentiation there is produced a body consisting of two 

 layers (a central and a peripheral one). But these two layers 

 pass through their further evolution in a somewhat different 

 manner than in the analogous processes of differentiation in 

 other animals. In the Cestoda the peripheral layer becomes 

 converted into a ciliary envelope (or its homologue), and the 

 central layer into a six-hooked embryo. From the researches 

 of E. van Beneden we obtain the data for a comparison of the 

 developmental history of the Tcenioi with that of the Botlirio- 

 cephali. This natui'alist has shown that after the e^g of the 

 Cestoda (both Teenies and Bothriocephali) has passed through 

 a J/or?Ja-stage (mulberry form), it becomes differentiated into 

 two layers, peripheral and central t- (Similar processes had 

 been previously observed in the Bothriocephalidai by KoUiker, 

 Mecznikoff, and Knoch.) These two layers are then developed 

 in different ways : from the outer one is formed, in the Bo- 

 thriocephalidae, the embryonal envelope (in the Tfenioi it en- 



• Properly speaking a differentiation has already taken place here, in- 

 asmuch as the peripheral cells have the cilia, which the central ones do 

 not possess. But this differentiation is essentially distinct from that of 

 the Cestoda and other animals, and it does not lead to the formation of 

 the germ-lamellse. 



t Recherches &c.,in M^moires couronn^sde I'AcAd. Royale de Belgiqiie, 

 tome xxvi. 



Ann.d' Mag. N. Hist. Ser. 4. Vol xv. 2 



