218 Mr. O. L. Simmons on the 



except in size and number of lamella}. The embryo hatches 

 in from seven to eight days after the stage figured in 10. 



With the reversion of the embryo the changes rapidly 

 proceed toward the adult condition. In fig. 7 I insert an 

 illustration which serves to connect my account with the 

 papers of Locy, Kishinouye, Laurie, and others. Here the 

 gill-lamellae have slightly increased in number while they 

 have become greatly increased in length. In the figure the 

 pulmonary sac is somewhat funnel-shaped, owing to a pulling 

 open of the spiracle in some process of manipulation. 



From this stage the transition to the conditions described 

 by MacLeod and Locy is but slight ; and although I have 

 studied the later stages up to and even beyond hatching, my 

 observations are but a confirmation of theirs, and so I do not 

 repeat them here. The lungs are well developed and appa- 

 rently ready to function as respiratory organs at the time of 

 hatching. With the growth of the young spider the principal 

 changes are an increase in the number of lamellae and a 

 corresponding increase in the size of the pulmonary organ, 

 the new lamellae being formed at the inner end of the sac. 



Tracheae. 



The study of these has been a matter of considerable diffi- 

 culty, and I have been able to follow with certainty only the 

 earlier stages. The tracheae arise behind the appendage on 

 somite IX, which, in its earlier stages, has exactly the same 

 history as appendage vm. There is the same inpushing 

 behind the limb, which results in its taking a position not 

 pointing outward, but towards the median line and backward. 

 Jn fig. 8 is seen the first differentiation of the tracheae. The 

 inpushing has given rise to the spiracle as before, but the sac 

 which results does not show so markedly those infoldings of 

 the appendicular wall which occur in the case of the lungs. 

 There is at most but a slight undulation of this surface. At 

 the inner end, however, two ingrowths are seen, the earliest 

 indications of the formation of the tracheal twigs. It is, 

 however, easy to see that these inpushings are to be compared 

 with the infoldings which produce the lamellae, while the 

 undulations just referred to admit of the interpretation that 

 they are aborted lung-leaves. 



After the reversion of the embryo the same parts can be 

 recognized (fig. 9). The inpushing has been carried to a 

 greater extent, and sections in other planes show that this 

 ingrowth is tubular in character. The cells lining its walls 

 are elongate and are already taking the character shown in 



