THE BLOOD-VASCULAR SYSTEM AND THE BODY-CAVITY. 87 



increase considerably in size, the primitive segments extending 

 towards the dorsal side (Fig. 45 B). This process exactly corre- 

 sponds to that we have already met with in the formation of the 

 coelom in the Annelida (Vol. i., p. 289). 



The following are the derivatives of the primitive segments : — 



1. The somatic layer gives rise to the body -musculature (as 

 thickenings near the ventral middle line of the abdomen), the two 

 strong longitudinal muscles being specially noteworthy, and also 

 to the subcutaneous connective tissue. According to Schimkewitsch, 



.the endoskeleton also is derived from the somatic layer, but this 

 statement Ave give with reserve. The covering of the parts arising 

 through invagination of the ectoderm (stomodaeum and proctodaeum, 

 lungs, glands), together with their musculature, thus the strong 

 musculature of the stomodaeum, already mentioned, is also derived 

 from the somatic layer. 



2. The splanchnic layer gives rise to the covering of the enteron, 

 the blood-vascular system, and the genital organs. 



The coxal glands are probably related to the mesoderm and coelom, 

 as in the Scorpiones (pp. 24 and 92), [cf. Brauer and Purcell]. 



G-. The Blood-vascular System and the Body-cavity. 



The Blood-vascular System. At a time when the limbs have 



already developed, there appear, above the primitive segments, 



between the ectoderm and the yolk, large round cells (Fig. 45 A 



and B, bl), concerning whose origin there is considerable difference 



of opinion. 



Balfour derived these cells from the yolk-cells. To the latter he also traced 

 the origin of the dorsal mesoderm (Fig. 29, p. 53). This last assumption was 

 refuted by Schimkewitsch, Locy, and Morin, who agree in stating that the 

 primitive segments extend to the dorsal middle line. The cells which, at later 

 stages (Fig. 29), are found dorsally, as in Figs. 45 B and 46, therefore belong 

 to the primitive segments. But, besides these, there are the large round cells 

 mentioned above (Fig. 45 A and B, bl), and with regard to their origin, 

 Schimkewitsch and Locv agree with Balfour, deriving them from the 

 yolk-cells. Kishinouye has recently adopted the same view, which seems 

 in accordance with the constitution of these cells. They are much larger than 

 the cells of the primitive segments (Fig. 45 A and B) ; we might, nevertheless, 

 like Morin, derive them from these, and assume that they had separated from 

 the primitive segments in an early stage, when the cells of these segments were 

 themselves larger. Better nourishment near the yolk as the cells increased 

 in number would also determine increase in size. This view is further supported 

 by the fact that they are found in the cavities of the primitive segments 

 (Schimkewitsch). This latter author, indeed, thinks that they reach these 

 cavities from the yolk by breaking through the wall of the segment, but this 

 view seems improbable. 



