Cleavage, Blastulation and Gastrulation 



219 



components of the egg (including yolk 

 spheres, mitochondria, neutral red-staining 

 granules, microsomes, etc.) are approxi- 

 mately uniform in their distribution in the 

 remaining space, except in the cortical layer 

 where the small jelly-precursor granules are 

 arranged to simulate alveoli. At fertilization 

 or activation, the jelly-precursor material is 

 extruded through the vitelline membrane to 

 the exterior, where it swells to form the 

 thick enveloping layer of jelly character- 

 istic of the fertilized Nereis egg. The germi- 

 nal vesicle ruptures a few minutes after 

 fertilization, releasing a quantity of nuclear 

 material into the cytoplasm. As the matura- 

 tion spindle is formed and tmdergoes its 

 divisions, there begins a slow movement of 

 hyaline cytoplasm and of cytoplasmic com- 

 ponents. The small spherical mitochondria 

 form a subcortical ring below the animal 

 pole. Most of these become incorporated into 

 the CD cell at the unequal first cleavage. 

 These bipolar movements of cytoplasmic ma- 

 terials continue slowly throughout the matu- 

 ration period and during the first two 

 cleavages. By the time the four-cell stage 

 is reached, ooplasmic segregation is very 

 pronounced. Most of the clear protoplasm 

 and smaller protoplasmic granules are to 

 be found in the animal hemisphere; the 

 neutral red-staining granules form a thin 

 cortical band just above the equator. The 

 oil droplets and yolk spheres occupy the 

 vegetal hemisphere, the oil droplets being 

 nearer the vegetal pole. During cleavage, 

 these are brought into contact and fuse with 

 each other in the foiu- entomeres, until, in 

 later development, only four remain, one in 

 each entodermal cell. Since these several 

 cytoplasmic inclusions take up vital dyes 

 differentially, the segregation is strikingly 

 apparent in vital-stained eggs (Spek, '34). 

 On the basis of this staining with indicator 

 dyes, Spek has concluded that the animal 

 hemisphere of the Nereis egg contains gran- 

 ules which are alkaline in reaction, in an 

 "alkaline" animal plasm. The vegetal hemi- 

 sphere correspondingly consists of an acid 

 yolk plasm, of which the yolk spheres are 

 the chief constituent. In the equatorial re- 

 gion are some acid granules in a meditun of 

 alkaline protoplasm. The so-called "alka- 

 line" region gives rise to the ectodermal 

 structures of the larva, the "acid" yolk-plasm 

 region produces the entoderm, and the border 

 region around the equator, containing acid 

 granules in "alkaline" protoplasm, gives rise 

 to the prototroch of ciliated ectoderm. 

 However, it appears clear that the sub- 



stances which take up the vital dyes in the 

 egg of Nereis dumerilii (Raven, '38), in the 

 egg of Aplysia (Ries, '39), and in the egg 

 of Nereis limbata (Costello, '36, '45) are 

 displaceable by centrifuging, if force of a 

 sufficient magnitude is applied (see Fig. 71). 

 It has been pointed out (Costello, '45) that 

 if the vital dyes are used at truly vital con- 

 centration, the hyaline protoplasm itself 

 does not stain, all coloration being associated 

 with granules or vacuoles. Many of these 

 inclusions approach the limit of microscopi- 

 cal visibility, but are movable by centrifugal 

 force if centrifuged for a sufficient period of 

 time with an appropriate force. The "acid" 

 and "alkaline" regions, therefore, are not 

 regions of acid or alkaline ground substance 

 (hyaline protoplasm), but are regions of 

 segregated granules or vacuoles which stain 

 differentially. So far as can be ascertained 

 by observation of stained and imstained eggs, 

 with visible or ultraviolet light, there are 

 no differences in the hyaline protoplasm 

 of the different regions. Any histogenetic sig- 

 nificance that could be attributable, even 

 theoretically, to the formed inclusions should 

 follow the induced pattern of these inclu- 

 sions in the strongly centrifuged egg. It is 

 well known from a multitude of centrifuging 

 experiments (Morgan, '08, '10; Lillie, '06, 

 '09; Conklin, '16; and others) that no such 

 histogenetic effect can be attributed to the 

 visible granules. Therefore, only localized 

 invisible differences within the hyaline pro- 

 toplasm itself could be of significance for 

 histogenesis. 



This conclusion that the visible cytoplas- 

 mic components of invertebrate eggs have 

 no morphogenetic value has been questioned 

 recently by Raven ('38) and Raven and 

 Bretschneider ('42). However, their objec- 

 tions to the conclusions of Conklin ('10) and 

 Clement ('38) appear to be without adequate 

 foundation, since they ignore the fact that 

 Clement obtained normal development of 

 hyaline fragments of centrifuged Physa eggs. 

 Harvey ('46) has obtained plutei from the 

 "clear quarter" of the Arbacia egg. 



A more complex type of segregation is that 

 found in the egg of the ascidian and de- 

 scribed by Conklin ('05). Upon fertilization 

 of the Styela egg, there is a primary segre- 

 gation of materials resulting from a down- 

 flow of the yellow and clear substances from 

 the animal toward the vegetal pole. This 

 active migration is completed within a few 

 minutes after sperm entrance. Then the 

 sperm nucleus moves to one side in the 

 lower hemisphere, inaugurating a secondary 



