THE BODY CAVITY. 



515 



In all other cases tliey are formed in a i-egion which appears to belong 

 to the epiblastic region of the cloaca ; and from my observations on Elas- 

 mobranchs it may be certainly couchxded that they fire formed thera 

 in this group. Tliey may appear as perforations (1) at the apices of 

 papillifoi-m prolongations of the body cavity, or (2) at the ends of cloacal 

 pits directed from the exterior towards the body cavity, or (3) as simple 

 slit-like openings. 



Considering the difference in development between the abdominal pores 

 of most types, and those of the Cyclostomata, it is open to doubt whether 

 these two types of pores are strictly homologous. 



In the Cyclostomata they serve for the passage outwards of the genera- 

 tive products, and they also have this function in some of the few Teleostei 

 in which they are found ; and Gegenbaur and Bridge hold that the primi- 

 tive mode of exit of tiie generative products, prior to the development 

 of the Miillerian ducts, was prol)ably by means of these pores. I have 

 elsewhere suggested that the abdominal pores 

 are peihaps remnants of the openings of 

 segmental tubes ; there does not however 

 appear to be any definite evidence in favour 

 of this view, and it is mf>re probable that 

 they may have arisen as simple perforations 

 of the body wall. 



Pericardial cavity, pleural cavities, 

 and diaphrag"!!!. In all Vertebrata the 

 heart is at first placed in the body cavity 

 (fig. 353 A), but the part of the body cavity 

 containing it afterwards becomes sepa- 

 rated as a distinct cavity known as the 

 pericardial cavity. JnElasmobranchii, 

 Acipenser, etc. a passage is however left 

 between the pericardial cavity and the 

 body cavity; and in the Lamprey a sepa- 

 ration between the two cavities does not 

 occur duriny; the Aminocoete stage. 



In Elasniobranchii the pericardial 

 cavity becomes established as a distinct 

 space in front of the body cavity in the 

 following way. When the two ductus 

 Cuvieri, leading transversely from the 

 sinus venosus to the cardinal veins, be- 

 come developed, a horizontal septum, 

 shewn on the right side in fig. 352, is 

 formed to support them, stretching across 

 from the splanchnic to the somatic side 

 of the body cavity, and dividing the body 

 cavity (fig. 352) in this part into (1) a 

 dorsal section formed of a right and left 

 division constituting the true body cavity 



Fig. 352. Section theough 

 the trunk of a scyllium embbyo 

 slightly younger than 28 f. 



The figure shews the separa- 

 tion of the body cavity from the 

 pericardial cavity by a horizontal 

 septum in which runs the ductus 

 Cuvieri ; on the left side is seen 

 the narrow passage which remains 

 connecting the two cavities. 



sp.c. spinal canal ; TF. white 

 matter of spinal cord ; pr. com- 

 missure connecting the posterior 

 nerve-roots ; ch. notochord ; x. 

 sub-notochordal rod. ; ao. aorta ;. 

 SI', sinus venosus ; cav.. cardinal 

 vein ; ht. heart; pp. body cavity; 

 pc. pericardial cavity ; ces. solid 

 oesophagus; Oliver; w^. muscle- 

 plate. 



33—2' 



