128 MALCOLM LAURIE. 



outer ends of the cells. Fig. 50 a shows a somewhat later 

 stage^ in which the cupping of the hypodermis has become 

 flattened out. There is no invagination of any sort, and the 

 eyes are, as Lankester and Bourne^ described them, mono- 

 stichous. The ventral nervous system has not undergone much 

 development. It has sunk somewhat deeper and is separated 

 from the hypodermis by the mesoblast. 



The tail has now developed its terminal segment — the sting. 

 The cavity of this last is partly occupied by the paired poison 

 gland, apparently formed by inpushing of the hypodermis (PI. 

 XVIII, fig. 51, /?. gl.). Each mass is connected to the super- 

 ficial hypodermis by a short duct. 



The gut extends down the whole length of the tail, and the 

 proctodseum is present in the form of a solid mass of hypodermis 

 cells blocking up its end (PI. XVIII, fig. 51, proct.). The gut 

 has also begun to grow forward (PI. XVIII, figs. 52 and 53). In 

 the last abdominal segment it is a complete tube surrounded 

 by a thin layer of mesoblast (fig. 52, int.). It gives rise to 

 two tubular outgrowths from its dorsal side, which are the 

 Malpighian tubes (fig. 52, mlph.). These run first towards the 

 back and then bend forward. There can be no doubt as to 

 their hypoblastic origin in this form, as the proctoda^um is not 

 yet formed. They have been already shown to be outgrowths 

 of the mesenterou in some Spiders by Loman,- and also in ter- 

 restrial Amphipoda by Spencer.^ Further forward (PI. XVIII, 

 fig. 53, int.) the gut is simply a semi-cylindrical layer of hypoblast 

 supported by a string of mesoblast and open to the yolk on its 

 dorsal side. In the thorax it has not yet begun to form. 



The mesoblast is broken up into strings and bands. The 

 coelom is still pretty distinct in the abdominal region (PI. XVIII, 

 fig. 53, COS.), and the heart is a large thin-walled tube apparently 

 connected with both somatopleure and splanchnopleure. As 

 mentioned above, the genital tube is formed in the somato- 

 pleure in the seventh somite and is a portion of the ca3lom. 



* ' Quart. Journ. Micr. Sci.,' vol. xxiii. 

 ^ ' Tijdschrift der nederl. Dierk. Vereen,' i. 

 ^ • Quart. Journ. Micr, Sci.,' vol. xxv. 



