THR DKA'ELOrMENT OF FALUIJINA VIVIPAKA. 115 



condition as in tlie previous stage. The pericardial septum 

 has disappeared. 



Stage F (fig. F and fig. 15). — All the essential features 

 may be seen in a view of the left side of the entire animal 

 (fig. F). Comparing this with the similar view of the pre- 

 ceding stage we find that there has been very rapid growth 

 in all parts of the body, especially in the "neck" region 

 between the viscei'al hump and the head. The bulge con- 

 taining the stomach and liver now lies nearly ventrally, 

 the mantle cavity has extended over into the left side of 

 the body, and just posterior to it is seen the pericardium, 

 with the heart now well developed, and showing the auricle 

 and ventricle separated from each other by a deep constric- 

 tion. The rectum lies higher on the right side than in the 

 last stage, and runs along the roof of the mantle cavity to 

 open more anteriorly. Fig. 15 is a transverse section across 

 the visceral hump of an embryo of this stage, and shows 

 essentially the same relation of the organs to each other as in 

 previous stages. The pericardium lies dorsally to the liver 

 and stomach, and contains the heart. Dorsal to the peri- 

 cardium are the two kidneys, the morphologically right, a 

 well-developed but still simple sac, being seen at k., its duct 

 at led. ] the opening of the kidney into the pericardium on 

 the one hand and the duct on the other are neither of them 

 shown in this section. To the right of the section is seen the 

 rudimentary left kidney [l. h.), which seems to resume its 

 development abont this time. Its opening into the peri- 

 cardium is well seen, and just above it is the solid end of 

 the original left horn of the mantle cavity [1. 7n, c). At 

 the extreme right (morphologically left) of the pericardium 

 the first rudiment of the gonad can just be distinguished 

 at gr.i 



A further important feature is the development of the 

 visceral connectives, which are first visible at this stage. 

 They arise anteriorly from the pleural ganglia and run back 



' For further description of the gonad and its connection with the rmti- 

 nieniary original left kidney, see Part I oftliis paper. 



