o Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 



entoderm. The lateral plates meet ventrally under the head about stage 23 but in the trunk region not 

 tili much later. 



Splanchnocoele. 



The splanchnocoele becomes patent first in the pericardiac region, the at first paired pericardiac 

 cavities soon becoming fused. The main part of the splanchnocoele develops by a Splitting process of the 

 lateral mesoderm spreading back from the pericardium and downwards from nephrocoeles and myocoeles. 

 In addition to the original communication between pericardium and the main splanchnocoele (pericardio- 

 peritoneal canals) there arises by Splitting a secondary more ventral communication. The pericardio- 

 peritoneal canals and the ventral communication become obliterated about stage 34—35 tlie latter a little 

 later than the former. 



Myotomes. (Graham Kerr, 1904.) 



In the early development of the myotomes there are two features of special interest and importance. 

 There is first the fact that the myoblasts of the inner wall of the myotome are for a time in the form ot 

 large neuromyoepithelial cells of the most diagrammatic looking kind — the cell being continued at its 

 inner end into a tail-like process — the motor nerve rudiment. The second feature of interest is that the 

 Dipnoans in question show beyond any possible doubt the development of muscle fibres from the external 

 wall of the myotome. The comparative certainty of observations of this in Lepidosiren is due to the fact 

 that the outer ends of the myoblasts of the inner wall of the myotome form a very characteristic broad 

 clear zone which demarcates in the most obvious way the inner wall from the outer wall. A relatively 

 considerable proportion of the substance of the definitive myotome owes its origin to the outer wall. 



Details as to the fate of the anterior myotomes, the musculature of the limbs etc. will be found in 

 Agar (1907). A detail of general morphological interest is the double origin of the constrictor pharyngis 

 (Wiedersheim) a typical "splanchnic" muscle in which as Agar has shown the dorsal part is actually 

 myotomic in origin. 



Nephridial System. 



The earliest indications of the kidney System make their appearance about stage 17 in the form of 

 a slight swelling of the mesoderm, producing a faint elevation of the dorsal surface of the embryo on 

 either side. The nephric rudiment so indicated gradually spreads backwards and about stage 22 a well 

 marked difference in size becomes apparent between the swoUen headward end of the rudiment which is 

 destined to become the functional pronephros, and the slender hinder portion which forms the pronephric 

 duct. About stage 24 the ducts are seen to have extended right to the cloacal region. 



As regards the details of structure in the earliest stages of development of the pronephros I do not 

 feel yet in a position to speak with any confidence owing to the extreme liability of error in investigating 

 the heavily yolk laden tissues. The pronephric rudiment in early stages forms a solid compact mass of 

 mesoderm which as seen in transverse sections forms a somewhat ellipsoidal mass projecting laterally from 

 the nephrotome between the ectoderm and the somatic mesoderm. Appearances point to the nephridial 

 rudiment being formed in the first instance by a series of nephrotomal outgrowths like those described by 

 Brauer for Amphihia, but solid instead of hoUow. As many as eight of these segmental tubule rudiments 

 are apparent in some Lepidosiren series stretching from myotome II backwards i). Except for these early 

 stages which until the technical difficulties in the way of their investigation have been completely overcome 



I) Traces of at least one tubule may appear in front of this. 



