THE EMBRYOLOGY OF A SCORPION. 107 



an elaborate description with numerous figures of the anatomy 

 of the adult. His description of the embryo is, however, very 

 brief and his figures unsatisfactory. 



Ellas Metschnikoff^ is the only writer who has treated of the 

 development of the Scorpion with any degree of fulness. He 

 gives a detailed account of the whole development, and his 

 paper, which deals chiefly with the surface views and optical 

 sections, contains a large amount of accurate and laborious 

 observation. It is the classic on this subject, and up to 1886 

 no attempt was made to add to it or supersede it. 



In 1886 Kowalevsky and Schulgin- published a short account 

 of the development of Audroctonus ornatus. Unfortu- 

 nately their paper has no figures, which detracts much from its 

 value. I find reason to differ from them on a few points, but 

 it is quite possible that this may be due to our having worked 

 on different genera. 



The only other paper on this subject which I am acquainted 

 with is by Gr. H. Parker,^ who treats at some length of the 

 development of the central and lateral eyes. I had worked at 

 this point before the appearance of his paper, and on the whole 

 agree with his conclusions. These are briefly that the lateral 

 eyes are monostichous, being formed from the hypodermis 

 without invagination. The median eyes, on the other hand, 

 are formed by invagination, and are therefore three-layered, 

 all the layers being derived from the hypodermis. The retina 

 is the second layer, the third being reduced to a post-retinal 

 membrane. The material at Mr. Parker's disposal did not 

 enable him to go back to the commencement of the formation 

 of the central eyes and their connection with the cerebral 

 invaginations. 



1 Metschnikoff, " Embryologie des Skorpious," ' Zeit. f. wiss. Zool.,' 1870. 



2 Kowalevsky and Scliulgiu, " Eutwickeluugsgeschichte des Skorpions," 

 'Biol. Centralblatt,' Bd. vi, 18S6. 



3 Parker, "The Eyes in Scorpioris," 'Bull. Mus. Comp. Zool., Harvard,' 

 vol. xiii. 



