ORIGIN OF THE REPRODUCTIVE GLANDS 



577 



through the retrograde development of the parts extending into the 

 appendages, through the development of the fat-bodies and through 

 the delamination of the somatic and the splanchnic mesoderm layer, 

 become greatly diminished in size. Finally, there remains left of 

 them only a rather small cavity (c), which is bordered on the side 

 by the rudiment of the pericardial septum (ps) and within by the 

 terminal thread-plate (ef). The dorsally situated point where these 

 two lamellae pass into each other seems to stand in intimate connec- 

 tion with the cells of the rudiment of the heart (h). The cord-like 

 genital rudiment 

 hangs from the ter- A /j \%\ 

 minal thread-plate '** 



as from a mesen- 

 tery (Fig. 549, gz). 



Together with 

 the growth of the 

 primitive band 

 around the yolk, 

 and the forma- 

 tion of the back, 

 the paired rudi- 

 ments of the heart 

 gradually extend 

 to the neighbor- 

 hood of the dorsal 

 median line, fol- 

 lowed by the geni- 

 tal rudiments 

 which are con- 

 nected with them 

 by the terminal thread-plates. The genital rudiments advance thus 

 to the dorsal side of the developing mid-intestine (Fig. 545, gz). 



The terminal thread-plate (ef) is at first a simple epithelial plate. 

 Soon, however, follows an arrangement of its cells whereby they 

 appear to be arranged in vertical rows, each one of which corre- 

 sponds to a developing ovarian tube. In this way the terminal 

 thread-plate separates into the separate terminal threads of the ova- 

 rian tubes (Fig. 550, ef). In this process of division the uppermost 

 dorsal edge of the terminal thread-plate takes no part. From it 

 afterwards grows a thread which extends anteriorly, which becomes 

 the common terminal thread of all the ovarian tubes, the so-called 

 Mtiller's thread. This is originally united with the pericardial 

 2p 



FIG. 550. Longitudinal section through the female genital rudi- 

 ments of P. ffermanica. A, with beginning, B, with farther ad- 

 vanced growth of the ovarian tubes : cz, rudiment of the genital effer- 

 ent passage ; ef, terminal threads ; ep, nucleus of the epithelial cells ; 

 gz, genital cells. After Heymons, from Korschelt and Heider. 



