UROCHORDA. 31 



from the same point of the embryo, but on the opposite side, a solid thick- 

 ening of epiblast which forms the rudiment of the nervous system. The 

 nervous system is placed close to the front end of the body ; and nearly at 

 the opposite pole, and therefore at the hind end, there appears immediately 

 below the epiblast a mass of cells forming a provisional organ known as the 

 elasoblast. Todaro regards this organ as mesoblastic in origin, and Salensky 

 as hypoblastic. The organ is situated in the position which would be 

 occupied by the larval tail were it developed. It may probably be regarded 

 (Salensky) as a disappearing rudiment of the tail, and be compared in this 

 respect with the more or less similar mass of cells described by Kupffer in 

 Molgula, and with the elaeoblast 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 comes 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 

 surface between the nervous system and the elaeoblast, 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 intercellular layer, which would seem to fill up the primitive 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 Todaro 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. 



