70 MARINE BIOLOGICAL LABORATORY. 



tern — takes place throughout the group, and the phe- 

 nomena are in some cases nearly as striking as in the 

 annelid types. 



What is the interpretation of concrescence ? Is it a 

 secondary adaptive mode of growth, necessitated by 

 some mechanical condition of development ? Or is it 

 a primary, ancestral process, which means that all un- 

 paired organs (such as the heart or the spinal cord) 

 now formed by concrescence were originally double ? 

 Both sides of this alternative have their adherents. 

 The first view, which is represented by very eminent 

 authority, regards concrescence as a process of resto- 

 ration (to use Professor Whitman's apt expression) by 

 which the two halves or the embryo, which have been 

 mechanically separated in the course of development, 

 are brought together again. I have not time to go fully 

 into the nature of the causes that are supposed to have 

 produced this separation. Broadly speaking, however, 

 the main cause is supposed to have been the excessive 

 accumulation of food (yolk) in the lower and middle part 

 of the egg, for the use of the developing embryo. This 

 mass of food, lying as it does in the median line, is sup- 

 posed to have temporarily bisected the embryo, as it 

 were ; so that a subsequent concrescence became a 

 mechanical necessity in the construction of the body. 

 According to the second view concrescence has a far 

 deeper meaning, though the probability is not lost sight 

 of that accumulation of yolk may have modified its char- 

 acter or heightened its effect. The origin of concrescent 

 growth is to be sought, from this point of view, in the 

 origin of bilaterality itself — an inquiry which brings us 

 to a deep-lying problem concerning the mode of deriva- 



