166 EMBRYOLOGY. 



transverse and frontal sections at the right and left of chorda 

 and neural tube, cubical sacs the walls of which are formed of 

 cylindrical cells ; these sacs are everywhere surrounded by a fissure- 

 like space, and they enclose a small cavity (the primitive-segment 

 cavity), which is a derivative of the body-cavity. From the front 

 layer of the fold is produced the posterior wall of the newly formed 

 segment, from its posterior layer the front wall of the remnant of 

 the primitive-segment plate, or of the sac which is next to be con- 

 stricted off. 



Of the Vertebrates which are developed out of meroblastic eggs, the 

 Selachians appear to exhibit most clearly the original mode of the 

 formation of primitive segments. A distinct body-cavity is formed on 

 either side of the trunk by the separation of the parietal and visceral 

 lamellae of the middle germ-layer (fig. 110). The dorsal portion of 

 the cavity, which flanks the neural tube, acquires thickened walls 

 (mp), and corresponds to the part previously designated as the 

 primitive-segment plate, which at the same time with the appear- 

 ance of the body-cavity begins to be divided into primitive segments. 

 In the anterior part of the body a series of transverse lines of 

 separation become visible (fig. 195 mp 1 ), the number of which is 

 continually increased toward the hind end of the body. For a 

 long time the cavities of the primitive segments, which are sepa- 

 rated from one another by these transverse furrows, remain in 

 communication ventrally with the common body-cavity by means 

 of narrow openings. One may therefore describe this state of 

 affairs by saying that the body-cavity is provided toward the back 

 of the embryo with a series of small sac-like evaginations, which lie 

 close together one after the other. Afterwards the primitive seg- 

 ments are entirely constricted off from the body-cavity, and then 

 their thickened walls come into close contact, and thus cause the 

 disappearance of the cavities of the segments (fig. Ill mp). 



Whereas in the Selachians it is still evident that the formation of 

 the primitive segments depends upon folding and constricting off, the 

 process is obscured even to obliteration in the case of Reptiles, Birds, 

 and Mammals; this is referable simply to the fact that the two 

 lamellae of the middle germ-layer remain for a long time firmly 

 pressed together, only subsequently beginning to separate, and that 

 they are composed of several layers of small cells. The process of 

 folding arid constricting off appears here as a splitting up of a solid 

 cell-plate into small cubical blocks. 



The part of the middle germ-layer that is next to the chorda and 



