HOUR-GLASS EMBRYOS. 



279 



B. Hour-Glass Embryos. Precisely the same kind of median fusion and 

 degeneration may appear at other points in the body, forming marked transverse 

 constrictions, or even complete fission. A common condition is where the con- 

 striction appears in the middle of the thorax forming the hour-glass embryos, as 

 in Fig. 189. 



Several other minor zones of transverse constrictions may be recognized. 

 They are formed at the points where a reduction in the size of organs and a tend- 



FIG. 185. Limulus embryos in various stages of degeneration; all drawn to the same scale. The process con- 

 sists in the union in the median line, and the subsequent degeneration, of the right and left organs of each metamere. 

 The organs nearest the median line are the first to unite, forming a single extra large organ having the characteristic 

 features of each member of the pair. The unpaired organ then decreases in size till it completely disappears. In 

 its place the organs lateral to it, on the same metamere, unite, and in turn degenerate; and so on till the whole 

 metamere has disappeared. The process begins in the most anterior metarneres, and progresses in a cephalo- 

 caudal direction. In typical cases, as soon as the first unpaired organ formed in, say, the first thoracic metamere, 

 has disappeared, the same organ is found unpaired in the second metamere; and by the time that has disappeared, 

 the unpaired condition of that organ obtains in the next following metamere; and so on, till every paired organ has 

 become median and unpaired, and then disappeared. In the later phases of the process, nothing is left of the 

 embryo but the mesodermic area and a posterior unpaired process, representing either the last thoracic appendage, 

 or the tail lobe; and these in turn finally disappear. 



In very rare cases, one of the posterior pair of appendages may fuse in the median line, without any indication 

 of fusion in front of that point. But there is no evidence of a progressive median fusion and degeneration extend- 

 ing toward the anterior end. Camera X30. 



ency to undergo median fusion takes place in the adults of other groups of 

 arthropods. They divide the body more sharply than usual into cephalic lobes, 

 oral, thoracic, vagal, and abdominal regions. Each of these regions, or tag- 

 mata, may show traces of degeneration from before backward, independently of 

 the others, and in the same manner that we have seen in the whole embryo. 



It would appear, therefore, that the broad subdivisions of the arthropod body, 



