OPHIURA BREVISPINA 433 



cytoplasm and its inner portion crowded with yolk spherules. 

 Then begins a process, which continues also during the invagina- 

 tion of the endoderm, of yolk segregation which is strikingly 

 similar in certain respects to that found in Ophiura. The inner 

 ends of the cells which contain the greater part of the yolk ma- 

 terial become constricted into the segmentation cavity. The 

 constricted ends of the cells contain no nuclei and are not split 

 off from the cells by or during mitosis. They soon disintegrate 

 into their component granules and spherules, thus forming a 

 general mass of free yolk material. Then follows a process, 

 as gastrulation proceeds, by which the yolk material of the blas- 

 tocoele flows into the archenteron, and Appellof believes that 

 it passes between the entoderm cells as the latter gradually 

 move toward the ectoderm, thus obliterating the segmentation 

 cavity. 



Appellof's interpretation of the significance of this process 

 of yolk segregation in Urticina is the same as that I have given 

 to it in Ophiura, the large amount of yolk in the cells of the 

 developing organism offers a very considerable hindrance to 

 developmental processes in general and especially to those 

 that involve the folding of epitheha, such as gastrulation, and, 

 by separating the greater part of the yolk material from the cells, 

 the larva is emancipated from this hindrance. 



The differences in the processes as found in Urticina and 

 Ophiura seem to be only those conceivably due to differences 

 in egg organization and larval structure in the two species. 

 In Ophiura the yolk material of the" unsegmented egg is equally 

 distributed throughout the cytoplasm, whereas in Urticina it 

 is localized to a certain extent in the central part of the egg. 

 In Ophiura the segmentation cavity persists throughout larval 

 development and the greater part of the yolk material remains 

 within this cavity until absorbed, a small portion only of the 

 yolk being found in the archenteron. 



Appellof refrains from discussing the significance which the 

 method of development found in Urticina may have in coelen- 

 terate development in general, but he suggests, not without 

 seemingly good ground, that the planula larva, which is typical 



