50 LILLIE. [Vol. X. 



olus. A surface section including these cells is shown in 

 Fig. 71. They are six in number, the nuclei large and trans- 

 parent, with usually a single very prominent nucleolus. Rabl 

 has figured these cells. A comparison of his Fig. 29 with 

 my Fig. 70 will leave no doubt as to their identity in the two 

 cases. They have, of course, been seen by Schierholz, who 

 has seen all that Rabl did. Rabl says of them (No. 25, p. 328): 

 "An der dem spateren Hinterende entsprechenden Korperseite 

 machen sich zu dieser Zeit drei durch ihre ausserordentliche 

 Grosse und ihre kugelige Form von alien anderen Ectoderm- 

 zellen auffallend abweichende Zellen bemerkbar"; and further 

 on, p. 370: "nach meinen Beobachtungen entsteht diese Driise" 

 (thread-gland) "durch eine, zwischen drei, am Hinterende des 

 Korpers gelegenen Zellen, auftretende Einstulpung des Ecto- 

 derms." He goes on to say that he cannot determine with 

 certainty whether or not these three cells are derivatives of 

 those mentioned before. It will be noticed that Rabl speaks 

 of three cells, whereas I have figured six. As a matter of 

 fact, three of these are more conspicuous in surface views 

 than the other three. 



According to my observations, Rabl is right in deriving the 

 thread-gland from the region of these cells, but wrong in 

 attributing its formation to invagination ; it is oftly one of 

 these cells zvhich forms the thread-gland proper, and that is the 

 one shown in the centre of the six (PI. V, Fig. 71). Though 

 smaller in superficial extent, this cell is really as large as those 

 surrounding it, for it is much deeper. The six cells in ques- 

 tion become more or less vacuolated ; the nucleus of the central 

 cell migrates to its inner end, the greater part of the proto- 

 plasm following it ; finally, almost all of the cell lies within 

 the primary body-cavity, but is still connected with the spot 

 it formerly occupied by a strand of vacuolated protoplasm. 

 The vacuoles run together, and form a lumen running from 

 near the nucleus to the exterior. Almost immediately a re- 

 fringent cuticular lining of the lumen is formed. The devel- 

 opment of the gland may be traced through Figs. 73 and 

 74. The other five cells of the complex later surround the 

 opening of the gland (Fig. %\\ cf. Fig. 43 of Rabl). These 



