500 T. GUSTAFSON 



It is for practical reasons suitable to denote the lowest part of the 

 blastula (the region at the vegetal pole) as zone i, the next as zone 2, etc. 

 These regions correspond to the presumptive ; 



1. primary mesenchyme 



2. secondary mesenchyme 



3. coelomic sacs 



4. oesophagus 



5. stomach 



6. proctodoeum 



7. ectoderm 



The films indicate that the morphogenetic activity in these zones can be 

 partly reduced to similar types of cellular activities. One of these is a 

 pulsatory activity of the cell surfaces bordering the blastocoel. The 



Fig. 2. Further example of the primary invagination {a and h) and of secondary 

 invagination (c). 



pulsations involve a centripetal translocation of cytoplasm, which causes 

 the cells to become thicker at their centripetal than at their centrifugal 

 ends. The pulsatory activity is often a forerunner of an emission of thin 

 pseudopods (filopods) from the inner cell surfaces. These pseudopods 

 either collapse or attach to the ectoderm and contract. A third change 

 involves a decrease or increase in adhesion between the cells in the 

 blastula or gastrula wall. These activities may be closely related to each 

 other. It looks, at least, that a vigorous pulsation can be modified into a 

 "shooting" out of pseudopods. 



I will try to give some examples how these three changes in cellular 

 activity co-operate in the morphogenesis of the sea-urchin larva : 



A. The pulsatory activity of the cells and their resulting change in 

 shape may bring about their release into the blastula cavity. This, how- 

 ever, only occurs if their adhesion to each other and to the hyaline mem- 

 brane, which covers the blastula surface, is low. The entrance of the 



