I05 



nor in any of the other cmbryos is tliere the slightest indication of the differcntiation of a 

 digestive epitheHum. The section passes on the dorsal side of the communication between the 

 axial mass and the anterior volk, which thus appear separate from each other. The intervening 

 tissue, which is in the main free from yolk-spherules, shews part of the membrane which has 

 ah-eadv been mentioned. The anterior jiart of the ectoderm consists of vacuolated cells with 

 refractive bodies, while the posterior two thirds is a good deal folded and consists of unvacuolated 

 cells which appear to be provided externally with long cilia. The posteriorly directed fold of 

 the collar-epidermis on the right side of fig. 210 cannot certainly be differentiated from the 

 other folds of the epidermis which are characteristic of the older embryos. But as a precisely 

 similar fold appears, a few sections further, on the other side of the embryo, with a similar 

 relation to the left collar-cavitv, I am inclined to resfard it as of some sigrnificance. 



Figs. 104 — 106 (PI. IX) are taken from a sagittal series of sections. Fig. 104, which 

 belongs to the lateral region of the embryo, shews the two transverse septa between the body- 

 cavities. Fig. 105 shews that not only is the posterior limit of the anterior body-cavity prolonged 

 into the yolk-mass as a region traversed by a definite line and free from yolk, but that a similar 

 yolk-free region corresponds with the division between the second and third body-cavities. This 

 is the case on the dorsal side at least. On the ventral side it is not certain that the coUar- 

 cavity exists. The ventral thickening resembles the anterior epidermic thickening of the adult 

 proboscis in the fact that the gland-cells are separated from the basement-membrane by a 

 homogeneous layer which in the adult has been interpreted as part of the epidermic nerve-plexus. 

 The dorsal posterior part of the epidermis is obviously ciliated in the section from which fig. 105 

 was drawn. 



In fig. 106, which is nearly median, the third body-cavity can be recognised both dorsally 

 and ventrally to the intestinal region, while the collar-cavity can only be made out dorsally. 

 This section shews the edge of an invagination {s. o.) of the dorsal vacuolated ectoderm of the 

 anterior end, which forms a conspicuous organ better seen in the series next to be described. 



Figs. 202 — 206 represent five sections taken from a transverse series of sections of an 

 advanced embryo, .stained with haematoxylin and Orange G. Fig. 202, through the anterior 

 end, shews the front part of the ventral thickening, the gland-cells of which, like those of the 

 adult proboscis of specimens treated in the same manner, are intensely coloured with the orange 

 stain. The dorsal vacuolated ectoderm, in which some of the refractive bodies are seen, projects 

 inwards in the form of a rounded organ (jr. o.) which has the appearance of a series of large 

 vacuoles separated by trabeculae coloured with haematoxylin. A crescentic section of the anterior 

 body-cavity (ó. c?) adjoins this organ, while the other space seen in the section is shewn, by 

 comparison with other specimens, to be external to the basement-membrane (<5.w.) of the epidermis, 

 and may be artificial. Part of this space remains at the base of the ventral thickening in fig. 203, 

 in which is seen the anterior part of the yolk, in the middle of which is an interval free from 

 yolk-spherules and traversed by a distinct line. This is the extreme anterior part of the division 

 between the two parts of the yolk shewn in fig. 210. The yolk-spherules are intensely stained 

 with Orange G, and the whole ma.ss is surrounded on all sides by the anterior body-cavity, in 



SIBOG.A-EXPEDITIE \X\l//lS. 14 



