WAITE: ANTENNAL GLANDS IX HOMARUS AMERICANUS. 181 



third region is along the entire length of the ectodermic sac in the dorso- 

 inedian area. An idea of the position of this region may be gained from 

 Figure 31 (Plate 3), which is a transverse section. It is there repre- 

 sented by the long dorsal tract of the ectodermic sac, extending mediad 

 near the letters "sac. trm." 



The complications in the shape of the ectodermic sac arise from the 

 differential growth in these three regions. The ventral fork (Figure 30) 

 grows ventrad around the proximal face of the endsac, and then distad 

 between it and the wall of the appendage, meanwhile extending laterad 

 and mediad in the frontal plane. It is this extension that finally con- 

 nects with the endsac. The proximal fork has grown at the same time, 

 but extends more nearly in the axis of the appendage. Meanwhile the 

 dorsal extension has grown considerably dorsad and distad, and the dorso- 

 median evagination has extended horizontally. As a result of all these 

 growths the parts of the ectodermic sac have come to embrace the end- 

 sac on all sides, though not completely, for it is seen in the larval stages 

 that through a part of its extent the endsac lies against the ventral and 

 lateral walls of the appendage. 



All these growths and extensions of the ectodermic sac are evagina- 

 tions, and carry with them part of the original lumen, but they are in no 

 respect convolutions. The complicated lumen thus arising is suppressed, 

 as far as actual cavities are concerned, until the latter half of embryonic 

 life. In an embryo of 152 days (July 1st to December 1st) no actual cav- 

 ity is observable, only the line marking the meeting of apposing walls, 

 but in an embryo of 303 days (July 1st to May 1st) the lumen is an actual 

 cavity at almost all points. In stages intermediate between these the 

 lumen becomes more and more open as age advances. We thus see that 

 the distention of the lumen occurs in large part prior to the junction 

 (see p. 179) of the lumina of the endsac and ectodermic sac. 



(c.) The endsac does not suffer much complication during its embry- 

 onic growth, but it becomes much flattened and compressed between the 

 extensions of the ectodermic sac. The chief growth is laterad and dorsad, 

 and gives rise to the condition found in the first larva, where the endsac 

 lies in large part lateral to the ectodermic sac (Figure 40, Plate 5). 



(d.) Finally the lumina of endsac and ectodermic sac become contin- 

 uous (see p. 178), and the organ has then practically reached the 

 larval condition. 



vol. xxxv. — no. 7. 3 



