316 NIKOLAS KLEINENBEKG. 



beginning at the lateral margins of the furrow in both the 

 hemispheres, Avhich are thus transformed into sacs with 

 double walls. This form of the embryo is represented in 

 profile in fig. 8, and in front view of fig. 9, where the 

 relations just described can be easily made out. Each of 

 the compartments encloses a cavity {cd), which communi- 

 cates with a common space opening to the exterior by a 

 fissure, already much contracted, in fig. 9. The walls of 

 each compartment consist everywhere of two or more layers 

 of cells, a very distinct ectoderm {ec) and an endoderm {en) ; 

 besides this, there are at the opposite extremities of each 

 two mesoblast cells (em) and two rows of flattened cells 

 [mes). Each of the lateral cavities (cd) will form the 

 digestive cavity of an individual, their openings into the 

 common groove will each become a mouth, and the single 

 e^g will produce two worms. To come to the end at once I 

 will explain the manner in which the perfect separation into 

 two indiriduals is accomplished. It is very simple ; each 

 embryo rotates about the axis of the uniting cord towards 

 the side opposite the common aperture, and turns at the 

 same time a little on its own long axis, but in the opposite 

 direction to the movement of the other ; from the first 

 movement results the enlargement of the aperture and of 

 the common cavity, which leads to their complete separation 

 and the approximation of the sides of the two embryos, 

 united by the median cord in such a way as to leave them 

 nearly parallel with one another. 



The second rotation produces a want of symmetry between 

 the planes of the longitudinal sections ', that is to say, the 

 corresponding meridians of the two embryos intersect nearly 

 at a right angle. The point where the uniting cord holds 

 together the two embryos corresponds to their necks, since it 

 is between the cord and the oral apertures (which are now 

 much restricted and converted into narrow canals) that the 

 two cephalic lobes take their origin. 



In this union the two embryos, forming a rather monstrous 

 twin organism, remain for some time, growing] and develop- 

 ing and completing their internal organisation, turning 

 gently in the albumen, without at all impeding one another, 

 by the concordant action of their vibratile cilia, which have 

 been some time developed. But little by little the commis- 

 sure relaxes to such a degree that the least pull is enough to 

 break it, a circumstance which can hardly fail to occur when 

 the contractions of the bodies of the embryos begin. It 

 thus happens that the Siamese twins dissolve their too close 

 relationship, which had probably become a nuisance to each 



