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sense. He applies it to the whole of the coelom lying at the side of the 

 gut. As defined by me the term includes only the portion cut off by 

 the transverse septum. The space in front of this is always in open 

 communication with the anterior coelom, of which I regard it as forming 

 a part. Only in this anterior region is the ventral mesentery ever absorbed. 

 At a very early date the left posterior coelom becomes much more 

 bulky than the right one, and tends to sweep under it on the ventral 

 side. This process takes place whilst the left coelomic sac is comple- 

 tely closed, and has for its effect the bending of the ventral part of the 

 left transverse septum so that it becomes almost longitudinal. Shortly 



Left Transverse septum in 

 process of absorption 



Left posterior Coelom 



Anterior Coelom 



Eight Transverse septum 

 in process of formation 



Right posterior Coelom 



afterwards a perforation is formed in this longitudinal part of the left 

 transverse septum, putting the sac again in communication with the 

 anterior coelom. This occurs just before the right transverse septum 

 is complete so that for a brief space all parts of the coelom are again 

 in open communication with one another. I have no doubt that it is 

 this secondary dissolution of the left septum which has been mistaken 

 by Goto for an absorption of the longitudinal ventral mesentery. 



Figure (1) is an instructive section taken from a larva of about 

 this age. It is a frontal section taken near the ventral surface and it 

 shows the left septum in process of absorption, and the right one in 

 process of formation. The identity in the nature of the two transverse 

 septa is shown by their mode of formation and, we may add, by the 

 dientical position at which they are formed on each side; a point 

 shown by all good frontal sections. 



