354 Ralph S. Lillie 



The ventral portion of the second septum, which, like ali the 

 other septa behiud the first, is intimatcly related to the subintestinal 

 vessel, carries blood-vessels ^ (which are already laid down in the 

 larva of Piate 24, Fig. 36) from the subintestinal vessel to the ventral 

 body-wall of the second somite. These blood-vessels inark throughout 

 lite the originai position of the second septum; this structure has 

 thus nothing to do with the formation of the second diaphragra. 



The second and third diaphragms, which in the adult form the 

 posterior boundaries of somites 'ò and 4 respectively, are formed 

 independantly of the primitive septa, and represent entirely diffe- 

 rent structures from these — as shown by their method of forma- 

 tion, histological composition, and relations to internai organs. The 

 second diaphragm begios its appearance at the time of formation 

 of the eleventh or twelfth somite; and in most instances completes 

 its extension across the body cavity during the time of formation 

 of the next two somites. Piate 24, Fig. 36 represents the appearance 

 in sagittal section of a larva of 13 somites, showing the appearance 

 of the newly formed diaphragm under a low power. It will be 

 observed that its line of Insertion into the ventral body-wall coin- 

 cides with that of the primitive septum. The latter structure persists 

 for some time after the diaphragm is formed, and can be seen in 

 its originai position stretching diagonally upwards and backwards 

 to the subintestinal vessel at the anterior border of the stomaeb. 

 The diaphragm on the other band, extends in a perfectly transverse 

 direction completely across the body-cavity and is inserted into the 

 Oesophagus in a position a short distance in front of the stomach. 



The formation of the diaphragm takes place through the inter- 

 action of several dififerent and i)artly independent processes. At its 

 first appearance (in a larva of 12 somites). it has the form of a 

 purely membranous partition, inserted into the ventro-lateral regioii 

 of the body-wall ou a level with the primitive septum, and cxten- 

 ding transversely to the Oesophagus which it joins at a regiou 



1 Blood-vessels of essentially similar relations are later laid down in the 

 post-diaphra.i,Mnal septa; and, as will l)e shown later, the distribution of the 

 scf^mental bliKjd-vessels in the adult and especially their peculiar relations to 

 the nephrostomata, are explained through the fact of their formation in the 

 primitive septa, where they appear as spaces between the two lamellae (Plate22, 

 Fig. 9; Piate 24, Figa. 32, 33). Since the nephridia are from the first in inti- 

 mate connection with the S('pta, the position of the nephrostomial blood-vessels 

 thus becoraes readily intelligible. These relations will be more fully dealt with 

 in the division trcating of the development of the nephridia. 



