26 SALPA. 



compared in this respect with the more or less similar mass of cells 

 described by Kupffer in Molgula, and with the elteoblast in Pyrosoma. 



After the differentiation of these organs a cavity makes its appearance 

 between the epiblast and hypoblast, which is regarded by Salensky as the 

 body cavity. It appears to be equivalent to the segmentation cavity of 

 Todaro. According to Todaro's statements, it is replaced by a second cavity, 

 which appears between the splanchnic and somatic layers of mesoblast, and 

 constitutes the true body cavity. The embryo now begins to elongate, 

 and at the same time a cavity makes its appearance in the centre of the 

 hypoblast cells. This cavity is the rudiment of the branchial and alimentary 

 cavities: on its dorsal wall is a median projection, the rudiment of the so- 

 called gill of Salpa. 



At two points this cavity conies into close contact with the external skin. 

 At one of these, situated immediately ventral to the nervous system, the 

 mouth becomes formed at a later period. At the other, placed on the 

 dorsal siirface between the nervous system and the elseoblast, is formed 

 the cloacal aperture. 



By the stage under consideration the more important systems of organs 

 are established, and the remaining embryonic history may be very briefly 

 narrated. 



The embryo at this stage is no longer covered by the walls of the 

 brood-pouch but projects freely into the atrial cavity, and is only attached 

 to its parent by means of the placenta. The epiblast cells soon give rise 

 to a deposit which forms the mantle. The deposit appears however to be 

 formed not only on the outer side of the epiblast but also on the inner side; 

 so that the epiblast becomes cemented to the subjacent parts, branchial 

 sack, etc., by an intei-cellular layer, which would seem to till up the primi- 

 tive body cavity with the exception of the vascular channels (Salensky). 



The nervous system, after its separation from the epiblast, acquires a 

 central cavity, and subsequently becomes divided into three lobes, each with 

 an internal protuberance. At its anterior extremity it opens into the 

 branchial sack ; and from this part is developed the ciliated pit of the 

 adult. The nervous ganglion at a later period becomes solid, and a median 

 eye is subsequently formed as an outgrowth from it. 



According to Todai-o there are further formed two small auditory 

 (? olfactory) sacks on the ventral surface of the brain, each of them placed 

 in communication with the branchial cavity by a narrow canal. 



The mesoblast gives rise to the muscles of the branchial sack, to the 

 heart, and to the pericardium. The two latter are situated on the ventral 

 side of the posterior extremity of the branchial cavity. 



Branchial sack and alimentary tract. The first development of the 

 enteric cavity has already been described. The true alimentary tract is 

 formed as a bud from the hinder end of the primitive cavity. The remain- 

 der of the primitive cavity gives rise to the branchial sack. The so-called 

 gill has at first the form of a lamella attached dorsally to the walls of the 

 branchial sack; but its attachment becomes severed except at the two ends, 

 and it then forms a band stretching obliquely across the branchial cavity, 

 which subsequently becomes hollow and filled with blood corpuscles. The 

 whole structure is probably homologous with the peculiar fold, usually pro- 

 lunged into numerous processes, which normally projects from the dorsal 

 wall of the Ascidian branchial sack. 



