144 MARY J. GUTHRIE AND HOPE HIBBARD. 



forms. Grave, in Ophiura brevispina, describes the mesenchyme 

 cells as being pushed or squeezed inward by the coercive pressure 

 of the organism as a whole, though this is not the whole story of 

 the process. MacBride describes the migration of mesenchyme 

 cells in Asterias vulgaris thus: 'The 'wandering' of these cells 

 seems to be effected by their emitting long filamentous pseudo- 

 podia which span the blastoccele, and along these strands the 

 body of the cell glides like a drop of dew on a spider's web." 



Serial sections through an embryo figured in i8a, i8b and i8c, 

 show the mesenchyme cells much more numerous, and those 

 most recently budded off are compactly pressed together. In 

 Fig. 19 the cells have multiplied still more and the mesenchyme 

 lies close to that part of the wall from which it arose. Fig. 20, 

 a stage still later, shows the mass of mesenchyme cells much 

 larger but still compact. In this set of material this condition 

 was found one hour and forty minutes before the first indication 

 of gastrulation, namely, the flattening of the vegetal pole. 



When the process of gastrulation begins, the mesenchyme cells 

 migrate laterally and become arranged in a ring in the blastoccele 

 about the polar axis of the embryo. The invagination of the 

 archenteron proceeds from the vegetal pole toward the space 

 unoccupied by mesenchyme. Fig. 21 is a very early gastrula 

 with the mesenchyme cells lateral to the invaginating arch- 

 enteron. A horizontal section through an early gastrula in 

 approximately the same stage as Fig. 21 is shown in Fig. 22. 

 The ring of mesenchyme is distinctly visible around the arch- 

 enteron. A later gastrula is shown in Fig. 23 in which the 

 secondary mesenchyme is being proliferated from the inner end 

 of the archenteron. The primary mesenchyme may be seen at 

 the sides of the archenteron. This specimen was somewhat 

 distorted in sectioning and for comparison, Fig. 24, drawn from 

 a camera lucida sketch of the living gastrula, has been inserted. 



Since the publication of Theel's paper, in which the literature 

 on the subject up to 1892 was discussed, the origin of the mesen- 

 chyme has been described for several echinoderms. Like the 

 earlier accounts, these, with the exception of those of Boveri 

 and MacBride, deal rather with the general question as to whether 

 the mesenchyme arises in the blastula at the vegetal pole, or in 



