414 EDGAR J. ALLEN. 



cede and follow it renders it higlily probable that there is a 

 complete closing of the cavity at this point, which is situated 

 in the anterior portion of the segment of the first maxillae. 

 Behind this point sections similar to that seen in fig. 14, d, 

 are met with, and this stage is specially important and inte- 

 resting. Two distinct cavities have here formed in the masses 

 of mesoderm-cells, and these cavities are separated from each 

 other by the cephalic aorta. After persisting throughout the 

 region of the first maxillse these cavities close, and the sections 

 appear as in fig. 14, e ; that is to say, solid masses of mesoderm 

 still lie on either side of the aorta. This condition of things 

 continues through the region of the second maxillse until the 

 point is reached at which, in the following stages, the dorsal 

 sac is found to end. This point is easily determined, because 

 the intestine is there attached by bands of connective tissue to 

 the dorsal surface of the carapace, the bands passing close to 

 the cephalic aorta. 



The most anterior region of the aorta and sac remains to be 

 considered. A transverse section through the parts, which is 

 nearly at the level of the mouth, is shown in fig. 14, a. The 

 appearance here presented is explained by fig. 15, which repre- 

 sents a horizontal section through the front part of the head. 

 The cephalic aorta, which has run horizontally from the heart, 

 suddenly bends downwards, and runs almost perpendicularly 

 towards the brain. It is surrounded during this part of its 

 course by the cells already described (fig. 12) as giving rise to 

 blood-corpuscles (/. g.), and has upon either side of it the two 

 muscles (c. m.) which are also found in the adult. The dorsal 

 sac is already open behind the aorta (fig. 15, c), and the trans- 

 verse section (fig. 14, a, c) passes along its length. 



The following numbers, obtained from the series of which 

 figs. 14, A — E, represent selected sections, will give an idea of 

 the extent of the difi"erent regions. The cavity of the sac is 

 seen below the aorta as in fig. 14, a, in three sections 

 at the level of the mouth, this condition passing into that of 

 fig. 14, B, where the cavity is below and on both sides of the 

 aorta, which continues for six sections. The next section is 



