498 T. GUSTAFSON 



One reason for this apparent difficulty to bridge the gap between the 

 molecular and the organ level is probably differences in language, con- 

 cepts, and slogans used by the workers in these two fields. Therefore, in 

 order to find a common point where anatomy and molecular biology can 

 meet, it seems logical to try to reduce the complex processes at the organ 

 level into morphological activities of individual cells. Could we, for 

 instance, show that the moulding of a certain organ shape depends upon 

 the formation of pseudopods, upon changes in the adhesive properties of 

 the cells, etc., then we could begin to discuss the molecular background for 

 these cellular phenomena. 



The translation of the phenomena at the organ level to a cellular 

 language is greatly facilitated by the use of time-lapse cinematography. 

 The sea-urchin larva is a suitable object for such a study, as it is transparent 

 enough to allows observations of the morphological activities of all its 

 individual cells. Furthermore, its anatomical organization is not too 

 complicated, and finally, much is known about the biochemical differentia- 

 tion of its cells. 



The application of time-lapse cinematography to the developing sea- 

 urchin larva raises some technical problems, as the larvae swim around. 

 We have, however, been able to overcome this difficulty by catching the 

 larvae in the meshes of a nylon net to which crystals of calcium carbonate 

 have been attached. The crystals make small indentations in the larvae and 

 keep them in a constant position without interfering with their normal 

 development [i]. I will try to give some examples of the results we have 

 obtained with this simple technique used in combination with conventional 

 time-lapse filming.* 



* The photographs in this paper are all reproduced from our i6-min. reversal 

 films of developing larvae of Psammechiuiis milioris. 



Fig. I. The formation of the primary mesenchyme and the archenteron in a 

 larva of the sea urchin Psamniecliimis miliaris. a-d show the release of the primary 

 mesenchyme cells which is brought about by a pvilsatory activity and the resulting 

 change in shape of the cells in combination with a decrease in their adhesion for 

 each other and for the hyaline membrane ; in d one of the cells has begun to form 

 a pseudopodium with which it migrates along the blastula wall (to the right). In e 

 the primary mesenchyme cells have settled down to form a characteristic ring-like 

 structure. / shows the end of the primary invagination of the archenteron, which 

 is brought about by pulsatory activity and the resulting change in shape of the 

 cells at the archenteron tip ; it also shows the first sign of the formation of a pseudo- 

 podium (protuberance to the left on the archenteron tip), g and h show diflferent 

 stages in the secondary phase of invagination of the archenteron, which is brought 

 about by the contractile pseudopodia formed from the archenteron tip. The time 

 covirse of invagination is shown in Fig. 3. Time interval between a and li 6 hr. 48 

 min. Picture series from a single i6-mm. time-lapse film. Magnification 430 x . 



