PROBLEMS OF ORGANIZATION 1 89 



all this may occur from a fragment which contained neither maternal 

 nor paternal chromosomes. There is a limit to development, and while 

 there is a preliminary organization, development ceases shortly after 

 the blastula is formed. In the ('46) contribution the clear quarter con- 

 taining the nucleus was fertilized after centrifugation and regular 

 plutei were obtained. In this series of experiments the action of the 

 cytoplasmic granules was analyzed. She concludes that the matrix or 

 ground substance is the material of importance and that the cytoplasmic 

 granules can be ruled out as requisite to the continued development here 

 attained. The granules are reconstituted after the blastular organiza- 

 tion is reached but not from previously localized granular material. 



There is one important observation in Harvey's ('46) paper which 

 is not stressed in the interpretation but which may play an important 

 part in the organizational complex. When the clear quarter is fertilized, 

 soon after fertilization the fertilization membrane usually breaks with 

 extrusion of the oil cap. If, however, fertilization is performed, one to 

 three hours later development may be regular. It may be that time is a 

 requisite factor for the molecular reorientation necessary for the ade- 

 quate formation of the fertilization membrane. If this is caused to form 

 without the necessary cortical reorganization, the deficiency either 

 qualitative or quantitative in the surface molecular layer manifests 

 itself in a degree of fragility which causes the breaking of the layer 

 and the expulsion of the oil cap. The temporal factor permitting of 

 material organization seems here to be of tremendous import. 



While the step by step analysis of the parts of Arbacia eggs has pro- 

 ceeded, it has shown clearly that the capacity of the embryo to undergo 

 its primary organization can be completed under rigorous experimental 

 ablation of many of its components. Certain combinations are necessary 

 for a complete development of the pattern but the pattern itself can be 

 broken down into parts which can be separated in time and action into 

 interrelated series, all combining in the interaction necessary for com- 

 plete embryo formation. 



One clear example of the influence of nucleus in development is 

 shown in the inverse symmetry of snails (Boycott and Diver ('23) 

 and Diver, Boycott, and Garstang ('25)). It will be remembered that 

 they studied a group of left-spiraled or sinistral types with which they 

 conducted experiments breeding the dextral type to the sinistral type 

 and securing its results in the offspring which indicated that there was 

 a definite recessive present so that the genetic results appeared not in 

 the first generation of crossing but in the second generation. There is 



