66 



MISCELLANEOUS VOTES. 



The line of cell-proliferation from the blastopore lips of these 

 parts is the prinvitwe streak. In Chelonia, and Clemmys, a groove 

 is distinct on the streak and represents the primitive groove ; in 

 Trionyx, it is absent, although sections show that cell-proliferation takes 

 place just the same. 



The anterior half of the primitive streak is much thicker than the 

 posterior half and gives rise to the mass known as the " Kndwulst." 

 The posterior half remains thin to the end. The anterior thick part 

 rises as the tail-swelling. This is formed by the addition of the new 

 cell-mass on the dorsal surface, in other words, by upheaval or elevation 

 and not by folding. 



When the tail has arisen to some height, the primitive streak and 

 groove should of necessity be found over the dorsal median line of the 

 tail, around its tip to the ventral median line, and then be continued to 

 the thinner part of the primitive streak outside the embryo proper, until 

 they reach the yolk-plug. They are thus 8-shaped. Such a condition 

 is actually seen in Chelonia up to quite a late stage. In Glemmys, the 

 anterior part of the streak and groove over the tail disappears rather 

 early. In Trionyx, the groove does not exist from the first, and the 

 streak also disappears early from the tail part. 



In the tail which lengthens itself backwards, the medullary chord, 

 the notocbord, the enteron, and the mesoblast are differentiated in situ 

 in the tissue surrounded by the epiblast. 



The primitive streak finally disappears entirely by the separation of 

 the layers, with the exception of a short stretch on the ventral surface 

 of the tail. The proctodeum is formed at this point. 



The yolk-plug, at the latest stage observed, stands out as an append- 

 age of the epiblast. Probably it persists to a late stage as a rudimentary 

 useless structure. In Glemmys, it is elongated posteriori}' and forms ou 

 the floor of the posterior amniotic tube a prominent ridge, the signi- 

 ficance of which is not clear. 



In Theoretical Considerations, the developmental processes as 

 above described are compared in detail with those brought out by 

 Schwarz, Sedgwick, the Zieglees and others in Elasmobranchii and 

 the identity of the so-called yolk-plug in Chelonia w ith the large yolk-mass 

 of Elasmobranchii is maintained. The conclusion is reached : " the 

 course of events described in flie preceding pages as talcing place in the 

 posterior portion of the embryonic area resulting in the formation of the 

 posterior part of the embryo in Chelonia is a repetition, in a rudimentali/ 



