Selected Invertebrates 



dermal part of the embryo can regulate to 

 such an extent that it produces derivatives 

 normally of ectodermal origin. 



Therefore the Tubifex egg is not as strik- 

 ing an example of specification of blasto- 

 meres by segregation of specific portions of 

 the cytoplasm into them as it was once 

 thought to be. Essentially similar results have 

 been obtained with the egg of Clepsine 

 (Leopoldseder, '31; Mori, '32), although cer- 

 tain differences in detail are encovmtered. 



CTENOPHORES 



Isolation, deformation and defect experi- 

 ments on the ctenophore egg (Driesch and 

 Morgan, 1895a, b; Fischel, 1897, 1898; Zieg- 

 ler, 1898; Yatsu, '12) give the following re- 

 sults, which indicate a progressive restriction 

 of the capacity to form swimming plates to 

 the first octet of animal micrcmeres and 

 which indicate that each micromere of the 

 first octet receives the developmental factors 

 necessary for formation of one row of swim- 

 ming plates. Each of the first two blasto- 

 meres, when isolated, develops into a larva 

 with four rows of swimming plates; each of 

 the first four blastomeres into a larva with 

 two rows of swimming plates; each of the 

 first eight blastomeres into a larva with one 

 row of swimming plates. If two macromeres 

 are removed in the 16-cell stage, such that 

 six macromeres and eight micromeres re- 

 mained, the resulting lai^va possessed all 

 eight rows of swimming plates. From this it 

 was conchided that the morphogenetic fac- 

 tors essential for formation of one row of 

 swimming plates had been segregated into 

 each of the first octet of micromeres (Fischel, 

 1897). But in the case described by Fischel 

 two of the rows of swimming plates were 

 much shorter than the others, and the pos- 

 sibility was not eliminated that this defi- 

 ciency was due to the lack of the second set 

 of micromeres in two octets, due in turn to 

 the absence of the macromeres (see Schleip, 

 '29). Yatsu ('12), however, seems to have 

 furnished evidence that only the first octet 

 of micromeres is concerned with formation of 

 swimming combs. If he separated two cells 

 of the 8-cell stage from the remaining six, 

 a larva with two rows of swimming plates 

 developed from the former. If, however, 

 he separated two macromeres together with 

 their two micromeres from the rest of the 

 blastomeres in the 16-cell stage, and then 

 removed the two micromeres, the two 

 macromeres formed a larva lacking swim- 

 ming plates. 



321 



Spek ('26) has demonstrated that a pro- 

 gressive segregation of corticoplasm into the 

 first octet of micromeres parallels this pro- 

 gressive restriction of potency during cleav- 

 age. This corticoplasm appears green in eggs 

 of Beroe under dark-field illumination. In 

 the fertilized but uncleaved egg it forms a 

 distinct and uniform layer over the egg. As 

 the first cleavage furrow develops the green 

 corticoplasm accumulates at the animal 

 pole, but as this furrow advances towards 

 the vegetal pole, much of the corticoplasm 

 is swept downwards towards the vegetal pole 

 and accumulates there temporarily. It then 

 becomes uniformly distributed again. These 

 shiftings of corticoplasm are repeated in the 

 second division and in the third cleavage, 

 except that in the latter the green cortico- 

 plasm remains as a cap at the animal pole 

 of each cell. At the following cleavage each 

 of these green caps is cut off almost com- 

 pletely into one of the first octets of animal 

 micromeres. Again a segregation of specific 

 portions of the cytoplasm into certain blasto- 

 meres is accompanied by a restriction of 

 certain developmental capacities to those 

 blastomeres.* 



TUNICATES 



The greatest variety of visibly distinct and 

 sharply localized cytoplasmic areas ever dis- 

 covered occurs in the egg of the tunicate 

 Styela. The progressive segregation of these 

 areas into the blastomeres is spectacular to 

 watch in living eggs (Conklin, '05a, b). Un- 

 fertilized eggs contain three protoplasmic re- 

 gions which are differently colored and are 

 distributed around the animal-vegetal axis 

 (Fig. 121/4). Following fertilization, striking 

 rearrangements of these regions occur, in- 

 volving a rapid flowing of the yellow proto- 

 plasm first to the vegetal pole (Fig. 1215), 

 then to the future posterior end of the egg 

 where it accumulates as a distinct yellow 

 crescent (Fig. 121C). The animal half then 

 contains clear protoplasm and the vegetal 



* But some of the green corticoplasm enters the 

 second octet of micromeres and some of it later 

 enters micromeres which arise at the vegetal pole, 

 and these cells are not involved in formation of 

 swimming plates. Moreover, not all derivatives of 

 the first octet of micromeres form swimming plates; 

 some form non-ciliated ectoderm between the eight 

 rows of swimming plates and between swimming 

 plates within each row. Thus only a part of the 

 green corticoplasm appears to be utilized in the 

 formation of swimming combs. Spek emphasized 

 that the green color of the comb-forming cells dif- 

 fered in no way from that in other ectodermal de- 

 rivatives. 



