Selected Invertebrates 



331 



levels. In other words, the whole egg may 

 now develop in a manner similar to the 

 original apical portion. Since the apical 

 region normally gives rise to ectoderm and 

 ectodermal derivatives, the whole egg may 

 now give rise to ectoderm alone, with neither 

 mesenchyme nor gut developing." 



The chief difference in the approach of 

 these two hypotheses has been an emphasis 

 on the specific effects of chemical treatment 

 on certain aspects of metabolism on the one 

 hand (Runnstrom-Lindahl) versus an em- 

 phasis on non-specific inhibition of vaguely 

 defined physiological activities on the other 

 (Child-Rulon), i.e., an emphasis upon kinds 

 of metabolism versus rates of activities. 

 More recently Rulon (literature summarized 

 '52) has attempted to characterize the ani- 

 mal-vegetal activity gradient in terms of 

 enzymes involved by exposing sand dollar 

 eggs to specific enzyme inhibitors. 



In commenting upon attempts to correlate 

 metabolism and enzyme activities with dif- 

 ferentiation, Holter ('49, p. 73) stated, "The 

 indications are that in the stream of chemi- 

 cal and metabolic events that constitute the 

 life of the embryo from fertilization to hatch- 

 ing, the true morphogenetic processes are 

 only like ripples on the surface and their 

 quantitative share in the chemistry of the 

 whole is very small. It seems rather doubtful 

 whether we can hope to reveal the mechan- 

 isms which cause those ripples by studying 

 overall metabolism and general enzyme dis- 

 tribution. We are obtaining very interesting 

 results as to the general biochemistry of the 

 egg and embryo, but the crucial problems 

 of morphogenesis may be beyond reach of 

 the enzyme chemist." In spite of this dis- 

 coiu-aging outlook some advance seems to 

 have been made towards analysis of deter- 

 mination of dorsoventrality in terms of en- 

 zyme activities. 



ESTABLISHMENT OF DORSOVENTRALITY 

 AND BILATERALITY 



Most animal eggs reveal earlier or later 

 a bilaterally symmetrical organization. 

 There is then recognizable a median plane 

 separating the egg into right and left halves. 

 In some echinoderms, for example the 

 holothvirians Psolus phantopus and Cucu- 

 maria frondosa (J. and S. Runnstrom, '21), 

 bilaterality of the egg is revealed visibly even 

 in oocyte stages. Much more commonly the 

 median plane stands revealed visibly and 

 becomes rigidly determined relatively late 

 in development compared with the axis of 



polarity. Nevertheless there is some evidence 

 both in tunicates and in sea urchins that 

 some steps in the determination of the 

 plane of bilateral symmetry have occurred 

 at very early stages. In the egg of the tuni- 

 cate Styela (Conklin, '05a) the sperm can 

 enter at any point within 30 degrees of the 

 vegetal pole; the sperm nucleus then moves 

 towards one side of the egg and much of the 

 yellow protoplasm located at that time at 

 the vegetal pole (Fig. 1215) is drawn over 

 with it, forming the yellow crescent just 

 below the equator (Fig. 121C). The center 

 of the yellow crescent indicates the future 

 median plane. The sperm nucleus often 

 travels through a considerable distance to 

 reach this side of the egg and Conklin 

 believes it is traveling towards a preformed 

 area, in other words, that the meridian 

 which becomes the median plane is already 

 fixed in the uncleaved egg. Certain experi- 

 ments on development of fragments of unfer- 

 tilized tunicate eggs (Dalcq, '38) likewise 

 suggested the possibility that bilateral sym- 

 metry is already determined in the tvmicate 

 egg before fertilization. Foerster and Orstrom 

 ('33) have presented physiological evidence 

 for the early existence of a dorsoventral 

 organization in fertilized (uncleaved) sea 

 urchin eggs. If such eggs are exposed to 

 potassium cyanide a smaller and a larger 

 hollow arise opposite each other; these hol- 

 lows later disappear, but if the position of 

 the larger one is marked by vital stain, the 

 resulting larva is stained ventrally. Such 

 an experiment suggests a dorsoventral dis- 

 tribution of some condition of the cytoplasm 

 which gives the same reaction to potassium 

 cyanide, but a more vigorous one ventrally. 

 Crowding of eggs, reducing availability of 

 oxygen, has the same effect. If such hollows 

 are suppressed by treatment with anionic 

 detergents (Gustafson and Savhagen, '50), 

 the resulting larvae are radially symmetri- 

 cal. Moreover, complementary deficiencies 

 which arise in meridional halves of unfer- 

 tilized sea urchin eggs also indicate that 

 a bilateral organization is already estab- 

 lished in this egg before the entrance of 

 the sperm (Horstadius and Wolsky, '36). 



There is also evidence that external fac- 

 tors can control the position of the median 

 plane. Just ('12) has demonstrated that the 

 first cleavage plane consistently passes 

 through the sperm entrance point in the egg 

 of Nereis (an annelid) ; the first cleavage 

 plane is perpendicular to the longitudinal 

 axis of the body. Just concludes (pp. 250- 

 251), "Since . . . the sperm may enter at 



