BODY-CAVITY 237 



still of minute size the organ shows a remarkable notching, of which traces are to 

 be found in the adult (fig. 170, p. 126 ; fig. 222, p. 176). 



The cells are at first closely packed, but spaces containing blood-corpuscles 

 appear, and the original cellular mass is converted into a trabecular framework. 

 The spaces become the venous sinuses of the organ. They are from the first 

 crowded by great numbers of leucocytes of all varieties. The artery is late in 

 developing. Hound its peripheral branches lymphoid cords are formed, which 

 become the Malpighian bodies. It is very difficult to determine the origin and fate 

 of the free cellular elements in the spleen, owing to the minute size and crowded 

 state of the cells in the higher vertebrates. The question whether they are 

 produced in situ, or introduced from without, can only be answered indirectly, 

 and opinion is divided on the point. 



In Lepidosiren the cells are of very large dimensions, and from observations on the 

 developing spleen the present writer v has been able to supply strong evidence in favour of the 

 view that the original mesenchyme-cells of the rudiment are differentiated into both red and 

 white blood-corpusdes, thus confirming the work of Laguesse and others on the Selachian 

 spleen. The spleen sinuses are at first merely spaces in the mesenchyme-mass, which later 

 become lined by endothelium, derived from the surface cells of the primary cellular trabeculae. 

 The spleen jn the lower vertebrates is thus an active hoBmapoietic organ. It is probably more 

 than a mere locus (i.e. a site in which the primitive corpuscles collect and multiply), as there 

 is evidence to show that blood-cells are actually formed from the indifferent cells of the rudiment. 

 In most mammals its share in blood-formation is limited to a certain period of foetal life, after 

 which that function is transferred to the bone-marrow. 



DEVELOPMENT OF THE BODY-CAVITY. 



The early stages in the development of the ccelom have already been described 

 on p. 49 seq. It was there explained that the coelomic space withinjthe embryonic 

 shield had at first the form of a U open behind, the clefts on each side' of the axis 

 being joined across the front of the shield by the precephalic ccelom. It was 

 also shown that, while inTthe region of the trunk the space was continuous with the 

 extra -embryonic ccelom, in the region of the head the future pericardial portion 

 was separated from it by a lamina of mesoderm. It was further explained that 

 when the head-fold is foi med the precephalic cleft comes to|lie below, then behind, the 

 bucco-pharyngeal membrane. It is by the expansion of this space that the definitive 

 pericardial cavity is developed, and in the following fashion. The pericardial 

 coalom consists at first of a mesial limb and two horns. The horns lie on either side 

 of the open pharynx, and their splanchnopleuric mesoderm is doubled in by the 

 primitive endothelial heart-tubes. At first on the ventral, these subsequently come 

 to lie on the mesial aspect of the splanchnopleuric folds as they bend in towards 

 one another. Before the folds meet to close in the floor of the pharynx they have 

 already become folded-in ventrally, and the mesial cross-portion of the pericardium 

 has come to lie below them, so that when their union is effected, and the heart- 

 tubes are brought together, there is no ventral mesentery (Robinson, Volker, 

 Rouviere). 2 By further extension backwards of the mesial cleft the lateral 

 horns are taken into the cavity, and the pericardial ccelom communicates with 

 the general ccelom only by two apertures, one on either side of the mesocardium. 

 When this disappears in part of its extent somewhat later, there is necessarily 

 only a single aperture extending across the middle line, and it further follows 

 that the lateral ccelomic spaces must also communicate in this situation with one 

 another below the free ventral edge of the gut-mesentery. 



1 Trans. Eoy. Soc. Ed. xli. 1904. 



2 Robinson, Jour, of Anat. and Phys. xxxvii. ; Volker, Bibliogr. Anat. x. ; Rouviere, Jour. d'Anat. et 

 de la Phys. xl. 



