DEVELOPMENT OP THE EMBRYONIC RUDIMENT LOLIGO. 25? 



yolk-sac may contract (KOLLIKER, METSCHNIKOFF) and, according 

 to BAY LANKESTER, carries out rhythmical movements which are 

 made possible by the fact that its envelopes do not consist, as is 

 generally supposed, merely of a layer of ectodermal cells and another 

 of entodermal cells, but also of a mesodermal layer intercalated 

 between these (Fig. 133) which evidently contains contractile elements! 

 This mesodermal layer, the presence of which in a number of other 

 Cephalopods (Sepia, Sepiola, Octopus, Argonauta) has been established 

 through the examination of sections, seems only slightly developed 

 in the forms which have smaller yolk-sacs, but in those with large 

 yolk-sacs as, for instance, Sepi((, this layer is highly developed, long 

 straight, fibre-like cells here lying one above the other. 



It appears that, by the contraction of the yolk-sac, its contents 

 are pressed into the interior of the embryo, extending far into the 

 embryonic rudiment (Figs. 132 and 133, p. 282). A distinction is 

 therefore made between the outer and the inner yolk-sac, the latter 

 extending as far as to the mantle and into the optic swellings. Here 

 also it is enclosed in the "yolk-epithelium" and does not, as has 

 been assumed, stand in direct communication with the intestinal 

 canal of the embryo, so that the yolk-substance can be utilised by 

 the embryo only with the help of the yolk-epithelium. As the yolk- 

 sac extends so far into the embryo, the surfaces are in contact over 

 a very large area, a fact which explains the absorption of the yolk 

 without direct communication existing with the intestine and with- 

 out the intervention of special vessels. The whole embryo (including 

 the yolk-sac) increases in size during development, the later stages 

 being much larger than the earlier. 



Although the great development of the external yolk-sac at first 

 recalls the conditions which prevail in the Vertebrates, some difference 

 is brought about in the Cephalopoda by the fact that the yolk-sac 

 is here devoid of any direct communication with the intestine. 

 Special vessels also seem to be wanting in the yolk-sac, as already 

 mentioned, and it therefore enters far into the embryonic body. 

 Further, the yolk-sac in the Cephalopoda is ventral, its position at 

 the anterior end of the body surrounded by the arms being merely 

 apparent. In reality, it lies ventrally (between the mouth and the 

 anus) and a comparison of Figs. 116 B and D, and 113, p. 248, 

 shows that the mouth lies at one side of the yolk-sac (the anterior 

 side) and the anus at the other (posterior) side. 



It has already been mentioned (p. 254) that the mouth appears as 

 a rudiment at an early embryonic period (Fig. 116 B). The anus 



