Description of the Notoehordal Invagination. 13 



of the canal appears definitely outlined, except at the extreme 

 dextral angle, where the process of disintegration seems to be still 

 widening and deepening the lumen at the expense of cells in that 

 region. Here nuclei have disappeared entirely from several cells, 

 being replaced by a finely granular substance. Other cells, scat- 

 tered here and there along the wall, or even within the degener- 

 ating area, possess the same granular contents. The greater 

 portion of the plug, however, is composed of completely empty 

 cornified cell walls. 



This process of degeneration so closely resembles the cornifica- 

 tion described by Unna (^76), Kusnetzofif (^67), and Gardiner ('84), 

 and that seen in my own preparations of the developing hoof and 

 Mullerian tegumentary gland of the embryo pig, that I have no 

 hesitancy in considering that the lumen of this portion of the noto- 

 ehordal canal, in the present instance, is being formed by a process 

 of either horny or fatty degeneration. It is difficult to distinguish 

 one of these forms of degeneration from the other. In the present 

 case there seems to be no evidence that resorption aids in the 

 formation of the posterior portion of the notoehordal canal. In 

 the anterior region, however, it is not impossible that the floor at 

 least disappears in this manner, since nuclei and cell walls disap- 

 pear at one and the same time, and are replaced by a sheet or 

 reticulum of granulated substance. 



It is interesting to note in the present instance that the anterior 

 portion of the lumen of the canal has been completed, and has 

 become confluent with the subgerminal cavity before the posterior 

 portion of the lumen is formed. 



Lieberkiihn ('82, p. 412) finds that in the case of the guinea-pig 

 the notoehordal canal is formed with a ventral entodermal, but no 

 dorsal ectodermal opening. In one instance he finds in the mole 

 ('81, pp.449 and 551) an indication of a dorsal opening. The 

 lumen therefore cannot, he says ('82, p. 412), be formed by invagi- 

 nation. The head-process of Cavia he finds in early stages to be 

 solid, or, when a lumen exists, it opens neither to the ectodermal 

 surface above nor through the entoderm below ('84, p. 436). In 

 an embryo with one protovertebra, the notoehordal canal opened 

 below for the space of eight sections ('84, p. 440). The notoehordal 

 cavity of the guinea-pig (Lieberkiihn, '82, p. 412) opened below by 

 means of a linear split. This split increased in size after its first 

 formation, as a study of older embryos showed. Kolliker ('82) found 



