408 



INVERTEBEATA 



CHAP. 



TEREBRATULINA SEPTENTRIONALIS 



Conklin did not himself collect and preserve the eggs and 

 embryos; these were collected for him and preserved in Perenyi's 

 fluid by Dr. Gardiner. Advantage seems to have been taken of the 

 spontaneous spawning of males and females when brought into the 

 laboratory. According to Morse, the egg, when laid, is slightly kidney- 

 shaped and about ^-th millimeter long, and, as seen by Conklin in 

 the preserved state, is oval 



The segmentation of the egg differs widely from that of any 

 Trochophore larva so far studied. It is true that it divides into 

 two slightly unequal blastomeres, then by a furrow at right angles 

 to this, into four, and then by a circumferential furrow into eight 

 blastomeres. It is true also that the " cross furrow," characteristic of 



B 



arch 



FIG. 322. Optical sections of early embryos of Tcrebratulina septentrionalis. 

 (After Conklin.) 



A, blastula, invagination of the vegetative pole just beginning. B, gastrula, invagination com- 

 plete : beginning of ridge which divides the archenteron into enteron and coelom. Arch, archenteron ; 

 bl, blastocoele ; Up, blastopore ; s, shelf which delimits enteron from coelom. 



spiral cleavage, is sometimes seen in the 4-cell stage. But the type 

 of cleavage is not constant, and it finally results in the formation of 

 a blastula in which there are a large number of cells absolutely 

 indistinguishable from one another, and in which it is impossible to 

 discriminate an animal from a vegetable pole. Sometimes in the 

 earlier stages of cleavage four larger cells are observed to be budding 

 off smaller ones, but the final result is always exactly the same 

 whatever the mode of cleavage ; in all cases a hollow spherical 

 blastula is formed which is ciliated all over. 



Although no experiments have been made to determine the 

 point, it seems fairly clear that, in the segmentation of the Brachiopod 

 egg, the cell divisions do not, as in Annelida and Mollusca, separate 

 organ-forming substances one from another, but that we have to do 

 with indeterminate cleavage. We shall become closely acquainted 

 with this form of cleavage when we study the eggs of Echino- 

 dermata. 



