514 FORMATION OF THE LAYERS. 



or from its lateral walls generally (Palaemon). They make 

 their first appearance soon after the invagination of the hypo- 

 blast has commenced. The mesoblast cells are at first spherical, 

 and gradually spread, especially in an anterior direction, from 

 their point of origin. 



According to Reichenbach there are formed in Astacus at the Nauplius 

 stage a number of peculiar cells which he speaks of as ' secondary mesoblast 

 cells.' His account is not very clear or satisfactory, but it appears that they 

 originate (fig. 237 F) in the hypoblast cells by a kind of endogenous growth, 

 and though they have at first certain peculiar characters they soon become 

 indistinguishable from the remaining mesoblast cells. 



Towards the end of the Nauplius period the secondary mesoblast cells 

 aggregate themselves into a rod close to the epiblast in the median ventral 

 line, and even bifurcate round the mouth and extend forwards to the 

 extremity of the procephalic lobes. This rod of cells very soon vanishes, 

 and the secondary mesoblast cells become indistinguishable from the 

 primary. Reichenbach believes, on not very clear evidence, that these cells 

 have to do with the formation of the blood. 



General form of the body. The ventral thickening of epiblast 

 or ventral plate, continuous with the invaginated patch already 

 mentioned, forms the first indication of the embryo. It is at 

 first oval, but soon becomes elongated and extended anteriorly 

 into two lateral lobes the procephalic lobes. Its bilateral 

 symmetry is further indicated by a median longitudinal furrow. 

 The posterior end of the ventral plate next becomes raised into 

 a distinct lobe the abdomen which in^Astacus at first lies in 

 front of the still open blastopore. This lobe rapidly grows in size, 

 and at its extremity is placed the narrow anal opening. It soon 

 forms a well-marked abdomen bent forwards over the region in 

 front (figs. 239 B, and 240 A and B). Its early development 

 as a distinct outgrowth causes it to be without yolk ; and so to 

 contrast very forcibly with the anterior thoracic and cephalic 

 regions of the body. In most cases this process corresponds to 

 the future abdomen, but in some cases (Loricata) it appears to 

 include part of the thorax. Before it has reached a considerable 

 development, three pairs of appendages spring from the region 

 of the head, viz. two pairs of antennas and the mandibles, and 

 inaugurate a so-called Nauplius stage (fig. 240 A). These three 

 appendages are formed nearly simultaneously, but the hinder- 

 most appears to become visible slightly before the two others 



