180 



Embryogenesis: Preparatory Phases 



and Schultz ('40) determined the ultravio- 

 let absorption spectra of different regions 

 within the nucleus and cytoplasm of a sea 

 urchin oocyte. Their results are shown in 

 Fig. 54. The curve for the nucleolus shows 

 an absorption maximum at 2600 A typical 

 of nucleic acid and a small rise at 2800 A 

 indicative of proteins containing aromatic 

 amino acids such as tyrosine and trypto- 



0.0 



2500 3000 A 



WAVE LENGTH 

 Fig. 54. Ultraviolet absorption spectra of different 

 parts of an ovarian egg of the sea urchin Psam- 

 mechinus miliaris (from Caspersson and Schultz, 

 '40). 



phan. The cytoplasm adjacent to the nu- 

 cleus shows a similar type of curve with 

 somewhat more protein indicated. The more 

 peripheral cytoplasm gives a rather different 

 curve indicative of considerably less nu- 

 cleic acid. The nuclear sap appears to con- 

 tain relatively little nucleic acid. This dis- 

 tribution of nucleic acid, which is in 

 accordance with cytochemical observations 

 of Brachet ('47), suggests diffusion of nu- 

 cleic acid formed in the nvicleus out to the 

 cytoplasm or diffusion from the nucleus of 

 some agent active in this synthesis, or syn- 

 thesis at the nuclear membrane. 



MATURATION DIVISIONS 



The details of meiosis are beyond the 

 scope of the present work and have been 

 presented in several texts (Wilson, '25; Dar- 

 lington, '37; Geitler, '38; Sharp, '43). A 

 few words may, however, be said here con- 



cerning possible causative factors. One strik- 

 ing feature of the phenomenon is that it is 

 essentially similar in cells that are so radi- 

 cally different in appearance as spermato- 

 cytes and oocytes. However, it is well known 

 that preparations for the meiotic divisions 

 are made at an early stage, shortly after the 

 last gonial division, when the germ cells 

 of the two sexes are more nearly alike in 

 appearance. These preparations involve the 

 intimate pairing (synapsis) of homologous 

 chromosomes, a phenomenon that does not 

 occur in the prophase of division of somatic 

 cells except in the Diptera. 



Among various suggestions as to the fac- 

 tors responsible for synapsis, the appeal to 

 antigen-antibody type of interaction first 

 made by Lindegren and Bridges ('38) seems 

 to be particularly cogent. Friedrich-Freksa 

 ('40) has interpreted the gene by gene spec- 

 ificity of pairing as due primarily to their 

 arrangement in similar order in the homolo- 

 gous chromosomes, which synapse by the 

 operation of nonspecific forces. However, the 

 preciseness with which the homologous 

 genes adhere, even when translocated or 

 inverted, would seem to require the inter- 

 vention of forces having the high degree of 

 specificity that is exhibited in antigen-anti- 

 body reactions. Lindegren and Bridges sug- 

 gest that the union is effected by the pres- 

 ence of agglutinins that are specific for 

 each chromomere of the chromosome. No 

 suggestions are made concerning such im- 

 portant details as to where and when these 

 antibodies are formed, or what reverses the 

 reaction in order to permit the chromosomes 

 to separate. 



Another interesting hypothesis to account 

 for homologus chromosome pairing has been 

 offered by Faberge ('42). This assumes the 

 operation of a hydrodynamical phenomenon 

 known as the Guyot-Bjerknes effect, in which 

 two spheres that pulsate in phase move 

 towards one another as a result of the 

 Bernoulli principle. The pairing units of 

 the chromosomes are considered to have the 

 properties of such pulsating spheres, each 

 with a characteristic frequency so that it 

 can attract its homologue but not non- 

 homologous units having other frequencies. 

 Faberge points out that the force is a long 

 range force such as seems to be required to 

 achieve synapsis between homologous chro- 

 mosomes that may be separated initiallv by 

 a distance of several micra in the nucleus. 

 Evidence for the operation of such long 

 range forces has been presented recently by 

 Hinton ('46) in a study of pairing of trans- 



