470 J. RUNNSTROM 



the marked region now consists of cells that are typical for a certain region 

 of the ectoderm. The cause of the injury was in the cases Fig. 2(6) and 

 2{c) the pretreatment with a low concentration of trypsin. This has 

 induced the activation of certain proteolytic enzymes probably of proteo- 

 lytic character in the egg. If the activation is limited or reversed normal 

 development occurs (Fig. 2(0)). If this does not occur the activation 

 continues and the proteolytic activity is such that the synthetic pathways 

 may be interrupted. It was concluded previously that the vegetal pathway 



An X -^.- >- Veg (q) 



D '■ 



An >^ • -*■ Veg (b) 



An X • *■ Veg (c) 



Fk;. 3. Diagram to illustrate the condition with respect to the vegetal and 

 animal vectors or pathways at the level of the square in the larvae represented in 

 Fig. 2. In (o) the vegetal pathways of synthesis dominate (Veg), but certain path- 

 ways of animal synthetic processes are also open. The four shorter lines represent 

 pathways that are common to all levels in the embryo. In (b) both the vegetal and 

 the animal pathways are interrupted. In (c) the vegetal pathways have been 

 reduced with a compensatory increase in the animal pathways which bring about 

 an animalization of the marked region. 



is more easily blocked than the animal one which is the cause of animaliza- 

 tion shown in Fig. 2(r). The animal or animalized cells are in general more 

 resistant to injury than the vegetal ones. The vegetal pathway gives the 

 impression of being more unstable.* 



* The recent results of Leone [13] are well in keeping with the views presented 

 above. He carried out a set of experiments on the effects of ribonuclease on 

 embryos of the sea urchin Arbacia lixiila. He found that the enzyme tends to 

 inhibit development in general and especially the differentiation of endoderm, 

 without animalizing the embryos. His results may be explained on the basis of our 

 diagram Fig. 3 {b). On a vegetal level both the animal and vegetal pathways are 

 interrupted by the treatment with the ribonuclease because the formation of 

 ribonucleic acid is the prerequisite for protein synthesis along both pathways. As 

 a consequence also the prerequisite for animalization of the vegetal region is 

 lacking. The somewhat greater resistance of the animal region of the embryo 

 found by Leone corresponds to the result illustrated by our Fig. 2(6). The latter 

 case may correspond to a stronger activation of hydrolytic enzymes in the vegetal 

 as compared with the animal region. On the other hand structural differences may 

 play a role in making the sensitive sites of the macromolecules in question more or 

 less accessible to the attack of the enzymes. The vegetal part of the embryo is also 

 more sensitive to disturbances in electrolvte composition of the medium, as to lack 

 of K+or SOr, cf. [21]. 



