UROCIIORDA. 33 



Development of the chain of sexual Salps. My description 

 of the embryonic development of Salpa would not be complete without some 

 reference to the development of the stolon of the solitary generation of Salps 

 by the segmentation of which a chain of sexual Salps originates. 



The asexual Salp, the embryonic development of which has just been 

 described, may be compared to the Cyathozooid of Pyrosoma, from which it 

 mainly differs in being fully developed. While still in an embryonic 

 condition it gives rise to a process or stolon, which becomes divided into a 

 number of zooids by transverse constrictions, in the same manner that part 

 of the germ of the ovum of Pyrosoma is divided by transverse constrictions 

 into four Ascidiozooids. 



The stolon arises as a projection on the right side of the body of the 

 embryo close to the heart. It is formed (Salensky, No. 35) of an outgrowth 

 of the body wall, into which there grow the following structures : 



(1) A central hollow process from the end of the respiratory sack. 



(2) A right and left lateral prolongation of the pericardial cavity. 



(3) A solid process of cells on the ventral side derived from the same 

 mass of the cells as the elaeoblast. 



(4) A ventral and a dorsal blood sinus. 



Brooks appear to be simply parts of the body cavity, and have certainly no connection 

 with the lateral atria of simple Ascidians or Pyrosoma. 



The observations of Todaro upon Salpa (No. 38) are very remarkable, and illustrated 

 by beautifully engraved plates. His interpretations do not however appear quite satis- 

 factory. The following is a brief statement of some of his results. 



During segmentation there arises a layer of small superficial cells (epiblast) and 

 a central layer of larger cells, which becomes separated from the former by a segmenta- 

 tion cavity, except at the pole adjoining the free end of the brood-pouch. At this point 

 the epiblast cells become invaginated into the central cells and form the alimentary 

 tract, while the primitive central cells remain as the mesoblast. A fold arises from the 

 epiblast which Todaro compares to the vertebrate amnion, but the origin of it is un- 

 fortunately not satisfactorily described. The folds of the amnion project towards the 

 placenta, and enclose a cavity which, as the folds never completely meet, is perma- 

 nently open to the maternal blood sinus. This cavity corresponds with the cavity of 

 the true amnion of higher Vertebrates. It forms the cavity of the placenta already 

 described. Between the two folds of the amnion is a cavity corresponding with the 

 vertebrate false amnion. A structure regarded by Todaro as the notochord is formed 

 on the neck, connecting the involution of the alimentary tract with the exterior. It 

 has only a very transitory existence. 



In the later stages the segmentation cavity disappears and a true body cavity is 

 formed by a split in the mesoblast. 



Todaro's interpretations, and in part his descriptions also, both with reference to 

 the notochord and amnion, appear to me quite inadmissible. About some other parts 

 of his descriptions it is not possible to form a satisfactory judgment. He has recently 

 published a short paper on this subject (No. 39) preliminary to a larger memoir, which 

 is very difficult to understand in the absence of plates. He finds however in the 

 placenta various parts which he regards as homologous with the decidua vera and 

 reflexa of Mammalia. 



B. III. 3 



