vm AKTHEOPODA 253 



Since the appendages of the acron which constitute the labrum 

 are situated in the mid-ventral line between the gun gl ionic enlarge- 

 ments, Hirsehler is inclined to deny their homology with the other 

 appendages, and to suppose that they owe their origin to a secondary 

 division of an originally unpaired outgrowth, such as gives rise to 

 the labrum in lower insects. 



We saw that, in an earlier stage, a gastral groove appeared in 

 the mid-ventral line and became closed in as a tube. Soon all trace 

 of a cavity in this tube disappears and the iuvaginated mass appears 

 as a more or less cylindrical rod, the upper end of which is wedged 

 into the yolk. For a while no clear boundary between it and the 

 ectoderm can be made out, but soon the invaginated mass is sharply 

 cut off from the ectoderm, first in the middle, then in the front part 

 of, and lastly in the hind part of the germinal streak. When this 

 has been accomplished the invagiuated rod flattens itself out into a 

 plate, remaining, however, thicker at the hinder end. Soon this plate 

 becomes differentiated into a median plate, which is the rudiment of 

 the endoderin, and two lateral plates, which are the rudiments of the 

 inesoderni. The median plate is only one cell in thickness, whereas 

 the lateral plates are each two cells thick ; both in front and behind, 

 however, the median plate is considerably thicker, and these regions 

 may be termed the anterior and posterior endodermic thickenings. 



Soon the mesoderrn of the lateral plates begins to exhibit a 

 moniliforni structure ; in a word, it is composed of thicker pieces in 

 the centre of each segment and of thinner portions below the grooves 

 which divide the segments from one another; that is to say the rnesoderm 

 exhibits on each side a segmentation into somites corresponding to the 

 segmentation of the ectoderm, marked out by the superficial grooves 

 dividing the segments from one another. The mesoderm of the acron, 

 or procephalic segment, is, however, merely a flat plate which passes 

 gradually into the thickened mesoderm of the antennary segment. 



Cavities now appear in many of these somites and the coelomic 

 sacs are thus established : they appear first in the thoracic and then 

 in the abdominal region, but none appear in the tenth and eleventh 

 segments of the abdomen. Still later a pair of sacs appear in the 

 segments belonging to the second maxilla, and a pair in the region 

 of the intercalary segment ; no other cavities appear in the head or 

 jaw regions. The thoracic sacs are small and round in section and 

 placed laterally near the points of origin of the lirnbs ; the abdominal 

 sacs, on the contrary, are oval in outline and extend almost to the 

 lateral borders of the segments. 



During this time the median plate, i.e. the endoderm, has also 

 been undergoing differentiation. The formation of the stomodaeum 

 as an imagination of the embryonic area has already taken place ; 

 this is situated at the level of the acron and is oval in shape 

 with the longer axis coinciding with the long uxis of the body. 

 About the same time a similar invagination appears at the level of 

 the eleventh abdominal segment and is the rudiment of the procto- 



