570 



EL A SMOBRA NGUI1. 



sp.c 



with it lies between the rnesoblast 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- 

 sonephros 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. 380, st) 1 , 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 



FIG. 386. SECTION THROUGH f the body cavity in the intermediate 

 EMBRYO 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 



tube; sd. segmental duct; sp.v. 

 spiral valve; r.subintestinalvein; 

 ^.o. primitive generative cells. 



THE TRUNK OF A 



SLIGHTLY YOUNGER THAN 28 F. 



sp.c. spinal canal; W. white 

 matter of spinal cord ; pr. poste- 

 rior nerve-roots; cli. notochord; 

 .r.sub-notochordal rod , ,10. aorta; end into the dorsal part of the body 



JmldS^a^y^nv^ted ^Y.^ <% 28 *> **>> aild fj 1 tube CUrl * 

 into muscles ; iv. rudiment of obliquely backwards round the inner and 



vertebral body ; st. segmental 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 



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

 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. 



