STRUCTURE AND DEVELOPMENT OF LOXOSOMA. 289 
From a comparison, however, of numerous sections of L. Lep- 
toclini and L. Tethyse at this and somewhat later stages, I 
have come to the conclusion that the blastopore closes in the 
position where the anus is subsequently formed, and that the 
oesophagus makes its appearance as a stomodseum in front oi 
this region. 
The mesoblast in Loxosoma originates in great part at 
any rate from the pair of pole-cells. In the later stages, as 
long as they are present, they are found in the immediate 
vicinity of the anus, each giving rise to a forwardly directed 
mesoblastic band. It is difficult by means of sections to deter- 
mine the period at which the anus originates, but even in the 
adult the anus can hardly be discovered except during the 
process of defsecation. I am unable to state positively how 
much of the alimentary canal is formed as a proctodaeum, but I 
regard it as probable that the whole of the “rectum,” as far as 
the constriction dividing the posterior limb of the alimentary 
canal into two parts (fig. 52), originates in this way; fig. 40, 
an, probably indicates the formation of the rectum as a proc- 
todseum. Whatever may be the real history of this part of 
the embryo, the intestinal end of the hypoblast seems to 
remain from the earliest stages in connection with the epi- 
blast, marking the spot where the blastopore either closes or 
persists as the anus. In front, however, occurs a large funnel- 
like epiblastic involution which forms the stomodseum, and 
early communicates with the arehenteron. In the embryo 
fig. 38 it is probable that the stomodseum extends as far as 
the line z, the histological difference between the (supposed) 
epiblastic and hypoblastic cells being very conspicuous. 
From a consideration of sections of L. Tethyse it appears 
probable that some of the mesoblast may originate entirely 
independently of the pole-cells from any portion of the embryo, 
but in later stages of L. Leptoclini the main production of 
mesoblast is probably due to the pole-cells, as indicated by 
fig. 40. This is a section passing through one of the meso- 
blastic bands formed from a pole-cell. The commencement of 
the band is at the side of the anus, as shown by the position of 
