UROCHORDA. 27 



On the completion of the gill the branchial sack becomes divided into a 

 region dorsal to the gill, aiid a region ventral to it. Into the former the 

 single atrial invagination opens. No gill slits are formed comparable with 

 those in simple Ascidians, and the only representative of these structures 

 is the simple communication which becomes established between the dorsal 

 division of the branchial sack and the atrial opening. The whole branchial 

 sack of Salpa, including both the dorsal and ventral divisions, corresponds 

 with the branchial sack of simple Ascidians. On its ventx'al side the 

 endostyle is formed in the normal way. The mouth arises at the point 

 already indicated near the front end of the nervous system', 



1 Brooks takes a very different view of the nature of the parts in Salpa. He says, 

 No. 7, p. 322, " The atrium of Salpa, when first observed, was composed of two broad 

 " lateral atria within the body cavity, one on each side of the branchial sack, and a very 

 " small mid-atrium.... The lateral atria do not however, as in most Tunicata, remain 

 " connected with the mid-atrium, and unite with the wall of the branchial sack to 

 " form the branchial slits, but soon become entirely separated, and the two walls of 

 " each unite so as to form a broad sheet of tissue, which soon splits up to form the 

 " muscular bands of the branchial sack." Again, p. 'd2i, "During the changes which 

 " have been described as taking place in the lateral atria, the mid-atrium has increased 

 " in size.... The branchial and atrial tunics now unite upon each side, so that the 

 " sinus is converted into a tube which communicates, at its posterior end, with the 

 " heart and perivisceral sinus, and at the anterior end with the neural sinus. This 

 " tube is the gill.... The centres of the two regions upon the sides of the gill, where 

 " these two tissues have become united, are now absorbed, so that a single long and 

 " narrow branchial slit is produced on each side of the gill. The branchial cavity is 

 " thus thrown into communication with the atrium, and the upper surface of the lat- 

 " ter now unites with the outer tunic, and the external atrial opening is formed by 

 " absorption." 



The above description would imply that the atrial cavity is a space lined by meso- 

 blast, a view which would upset the whole morphology of the Ascidians. Salensky's 

 account, which implies only an immense redueticm in the size of the atrial cavity as 

 compared with other types, appears to me far more probable. The lateral atria of 

 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 laj'er of small superficial cells (epiblast) and 

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

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

 the epiblast cells become invagiiaated 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 permanently 

 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 disappeai'S 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. 



