562 ON INCREASE IN COMPLEXITY [pt. iii 



have amoeboid properties ; they form an example of the first class. 

 Linear aggregation would be found in the formation of blood-vessels 

 and superficial aggregation in the formation of the yolk-sac in certain 

 mammals. Aggregation of cells into spherical masses is common in 

 organogeny, e.g. the origin of the spleen. The attachment of isolated 

 cells to another body takes place when tendon is united to bone, 

 and penetration by isolated cells into another body when the vitreous 

 humour of the eye is secreted. Phagocytosis plays a large part mostly 

 in post-embryonic differentiation such as the absorption of the tad- 

 pole tail, and perhaps in insect metamorphosis, but change of shape 

 in cells is usual in all embryos, i.e. from flat to columnar or from 

 neuron to neuron plus axon. 



The movements of linear masses are more important still; thus 

 growth in length is seen in the back growth of the segmental duct 

 during the formation of the somites. Splitting occurs at the end of 

 linear masses whenever they branch as nerves and vessels frequently 

 do, and throughout their length in, for instance, the elasmobranch 

 segmental duct. Anastomosis is very common within the circulatory 

 system, and fusion with other organs occurs in the case of the vasa 

 eflferentia and the mesonephros, as also in the case of the formation 

 of the pituitary body. The movements of superficial aggregates, 

 sheets or layers, involve various alterations, such as increase in area 

 in the growth of the chick blastoderm over the yolk or the formation 

 of the mammalian blastocyst. This increase in area may be uniform 

 all over the surface, in which case the structure at the end of the 

 process will have much the same form as it had at the beginning, 

 or it may not be, in which case invaginations and foldings will be 

 produced. The optic cup is produced in this way, as also is the 

 medullary groove. As will later be seen, this unequal expansion of 

 surface has important connections with the theory of metabolic rate. 

 The sheet of cells may also thicken as the mammalian trophoblast 

 does in the formation of the placenta, or it may become thinner as 

 the roof of the thalamencephalon and medulla does, or the outer 

 wall of the lens. If it goes still further in this direction, it may break 

 altogether and a perforation be formed. This is what happens to 

 the floor of the archenteron in the amniota, and in many other 

 instances. Again, two separate layers or sheets of cells may fuse into 

 one by a process of concrescence, and this is what happens when 

 the medullary folds close over the medullary groove uniting flatly 



