570 



EL A SMOBRA NCHIL 



S(I". 



I 'p.O 



with it lies between the mesoblast and epiblast. The knob and 

 column do not long remain solid, but the former acquires an open- 

 ing into the body cavity (fig. 421, sd) 

 continuous with a lumen, which makes 

 its appearance in the column (fig. 386, 

 sd). The knob forms the only structure 

 which can be regarded as a rudiment 

 of the pronephros. 



While the lumen is gradually being 

 formed, the segmental tubes of the me- 

 son ephros become established. They ap- 

 pear to arise as differentiations of the 

 parts of the primitive lateral plates of 

 mesoblast, placed between the dorsal end 

 of the body cavity and the muscle-plate 

 (fig. 386, sty, which are usually known 

 as the intermediate cell-masses. 



The lumen of the segmental tubes, 

 though at first very small, soon becomes 

 of a considerable size. It appears to be 

 established in the position of the section 

 of the body cavity in the intermediate 

 cell-mass, which at first unites the part 

 of the body cavity in the muscle-plates 

 with the permanent body cavity. The 

 lumen of each tube opens at its lower 

 end into the dorsal part of the body 

 cavity (fig. 386, st), and each tube curls 

 obliquely backwards round the inner and 

 dorsal side of the segmental duct, near 

 which it at first ends blindly. 



One segmental tube makes its ap- 

 pearance for each somite (fig. 265), com- 

 mencing with that immediately behind the abdominal opening of 

 the segmental duct, the last tube being situated a few segments 

 behind the anus. Soon after their formation the blind ends of the 

 segmental tubes come in contact with, and open into the segmental 

 duct, and each of them becomes divided into four parts. These are 

 (1) a section carrying the peritoneal opening, known as the peri- 

 toneal funnel, (2) a dilated vesicle into which this opens, (3) a 

 coiled tubulus proceeding from (2), and terminating in (4) a wider por- 

 tion opening into the segmental duct. At the same time, or shortly 

 before this, each segmental duct unites with and opens into one of 



Fig. 386. Section through 

 the trunk of a scyllium embryo 

 slightly younger than 28 f. 



sp.c. spinal canal; W. white 

 matter of spinal cord ; jor. jjoste- 

 rior nerve-roots ; ch. notochord ; 

 X. sub-notochordal rod ; ao. aorta ; 

 7«p. muscle-plate; inp' . inner layer 

 of muscle-plate already converted 

 into muscles ; Vr. rudiment of 

 vertebral body ; st. segmental 

 tube; sd. segmental duct ; sp.v. 

 spiral valve ; I'.subintestinalvein; 

 p.o. primitive generative cells. 



1 In my original account of the development I held these tubes to be invaginations 

 of the peritoneal epithelium. Sedgwick (No. 549) was led to doubt the accuracy of my 

 original statement from his investigations on the chick; and from a re-examination 

 of my specimens he arrived at the results stated above, and which I am now myself 

 inclined to adopt. 



