28 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



dermic strings extending from the mesoderm which invests the supra-cesophageal 

 ganglion to the outer amnion fold. 



Figure LXXXII represents a section of the same series close to the median line. 

 Here it will be observed that the inner amniotic fold has separated from the outer 

 and lies close to the supra-oesophageal ganglion, to which it is apparently attached. 

 Blood cells have broken down between the two folds and have thus formed blood 

 plasma and blood corpuscles. Blood cells are numerous in the dorsal surface of the 

 embryo, where they generally lie between the two layers of mesoderm, indicating 

 perhaps that the blood once circulated through the body cavity. Balfour describes 

 the groove regarded here as amnion, as a depression on the anterior and lower sur- 

 face of the procephalic lobes. Metschinkoff, (1) in his article on the embryology of 

 scorpions, has described a fold which closes in the ventral surface of the brain of the 

 scorpion, and is probably similar to the amnion of spiders. If this fold were 

 extended the whole length of the embryo it would correspond in every way to the 

 amnion of insects. 



The observations presented here on the embryology .of spiders can be very 

 briefly summarized. 



The primitive cumulus, consisting of undifferentiated cells, appears before the 

 blastoderm is fully formed. 



It probably forms a considerable part of the mesoderm. According to Balfour 

 it occupies a position where the caudal lobe of the embryo subsequently appears. 



The mesoblast, or part of it at least, must then grow forward from this posterior 

 part of the embryo. 



Amniotic folds appear in the head region of the embryo. 



In the species of spider studied, probably two abdominal appendages are invagi- 

 nated to form each lung book. 



The supra-cesophageal ganglion is indistinctly divided into two portions. 



CONCLUSION. 



It may be well to close this paper with some remarks on the relations of 

 tracheates suggested in part by the observations given. 



(i) EmbryoloRie des Scorpions. Zeit. f. wiss. Zool. Bd. XXI. 



