OPHIURA BREVISPINA 429 



cells composing the epithelia. Whether the yolk of the seg- 

 mentation cavity continues to be enclosed within the mesen- 

 chyme cells or whether it may come to lie free in this cavity 

 can not be determined from the figures. All of the yolk material 

 shown in the larval cavities seems to be satisfactorily accounted 

 for as belonging, or as having belonged, to the invading cells, 

 and evidence is wanting to show that yolk segregation in this 

 species is effected in any other way than by a great increase in 

 the number of mesenchyme cells produced. There is no evi- 

 dence that the yolk laden cells of the larvae, as a whole, extrude 

 yolk into the blastocoele and archenteron as is the case in 

 Ophiura. 



Solaster. According to Gemmill the yolky egg of Solaster 

 endeca has an equatorial diameter of 1 mm., a vertical diameter 

 of 0.8 mm. The gastrula has a large empty archenteron and 

 a small narrow segmentation cavity completely filled with 

 stellate mesenchyme cells. As development proceeds, however, 

 mesenchyme cells, consisting in chief part of large sized yolk 

 granules, continue to be budded into the coelomic spaces from 

 the basal aspect of the cells lining the anterior and posterior 

 coelomic pouches. All cells of the younger stages contain yolk 

 material, but, when stages of late metamorphosis are reached, 

 the cells forming the growing enteron of the larva contain at 

 least half of the remaining yolk, and from this fact it is inferred 

 that a gradual transference of yolk material from mesenchyme 

 cells to entoderm cells must have taken place. 



Here again we have a species in which mesenchyme cells 

 have assumed the function of secondarily receiving yolk material 

 from other cells, thus segregating it for a time and finally ef- 

 fecting its transference to other cells. 



Gemmill records observations on certain larvae, considered 

 by him to be possibly abnormal, which are of special interest 

 in connection with a consideration of the yolk segregation process 

 found in Ophiura. During the rearrangment of the cells of 

 certain blastulae of Solaster a few yolk granules were pressed out 

 of the cells and in the living specimens were seen eddying hither 



