Structure and Development of the Nephridia. 353 



the piane of Insertion of the retractor muscles of the proboscis. The 

 Oesophagus extends Ijackward throngh the seeond somite and joins 

 the stomach uear the anterior limit of the third somite; it is evi- 

 denti}' undergoing elougation, as may be seen by eomparing its 

 extension at tliis stage with tbat fouud in the earlier stages repre- 

 sented in Figures 7 and 8. This backward Prolongation is further 

 indicated in tlie fact that the septa, which in earlier stages occupied 

 planes approximately perpendicular to tlie long axis (Plates 22 and 

 23, Figs. 7, 8, 10), bave acquired a pronounced slant upwards and 

 backwards. It is to be carefuUy noted that at this stage the seeond 

 septum is inserted almost at the anterior boundary of the stomach, 

 while the third and fourth septa, whose line of insertion into the 

 body- wall corresponds with that of the later formed diaphragms, are 

 attached to the ventral blood-vessel at points far behind the anterior 

 boundary of the stomach. 



As the Oesophagus elongates, the stomach is, as it were, pushed 

 backwards, aud the above mentioned Ijackward slope of the septa 

 becomes more marked (Piate 24, Fig. 36). This backward growth 

 has the further result that when the seeond and third diaphragms 

 are formed (in a manner shortly to be described), each becomes 

 inserted luto the Oesophagus at a position well in advance of the 

 stomach, although as just showu, this organ originally extends far 

 in advance of the region finally occupied by the Oesophagus. The 

 stomach seems during these changes of position to undergo an ex- 

 tensive histolytic disintegration and reorganization to form the tissue 

 of the Oesophagus — a fact Avhich must also be taken into conside- 

 ration in accouuting for the changes in the position of septa and 

 diaphragms. These histolytic changes will shortly be described. 



The most anterior septum occupies from the first a position 

 immediately behind the line of insertion of the proboscidial retractor 

 muscles (Piate 24, Fig. 35], As growth proceeds, the relations of 

 these two originally independent structures become closer, and at 

 the stage represented in Fig. 36 — • in which twelve somites bave 

 been laid down — the two bave become intimately associated with one 

 another, although stili distinguishable on dose examination. This 

 association of })roboscidial muscles and first septum becomes in later 

 stages more complete, aud both eventually euter into the formation 

 of the muscular first diaphragm. The posterior peritoneal wall of 

 the adult diaphragm may be regarded as representìng the originai 

 first septum. . 



