EMBRYOLOGY OF THALASSEMA MELLITA 179 



Thalassetna. Podarke. 

 r Rosette 4 4 



_. I Cross 8 4 



First quartet. < x . ■ ^1 c c 



J Intergirdle S o 



V, Prototroch 16 16 



36 32 



Second quartet 12 16 



Third quartet 8 8 



Fourth quartet 4 4 



Macromeres 4 4 



28 32 



64 64 



2. Further Divisions of the Quartets. 



a. First Quartet. — From the first quartet subsequently 

 arises a considerable part of the ectomesoblast, but, as it is not 

 formed until after gastrulation, a description of the process is 

 reserved for another section. The following section deals largely 

 with the further divisions and fate of the cross and intergirdle 

 cells, which are of interest from a comparative standpoint. 



The hitergirdle Cells. — After the completion of the 64-cell 

 stage, the first division in the first quartet is an extremely un- 

 equal and dexiotropic one of the intergirdle cells in the a, b 

 and c quadrants (1^1,2.1.) etc.). Three minute and entirely 

 superficial cells (i^jojo. , etc.) with dark staining nuclei are 

 budded off toward the prototroch (PI. I, Fig. 4). As will be 

 shown later, these cells are rudimentary and in the end completely 

 disappear. In the d quadrant, on the other hand, the corre- 

 sponding division is nearly equal, with the spindle inclined very 

 slightly dexiotropically (PI. I, Fig. 4). The lower cell {\d 

 1.2.1.2.) soon divides bilaterally (PI. I, Figs. 6 and 7) and gives 

 rise to two cells, which lead the subsequent backward migra- 

 tion through the dorsal gap in the prototroch and into the 

 post-trochal region. Although these cells are not " homoblas- 

 tic " with, yet they appear to be "equivalent"^ to the cells 



^ These terms have been recently suggested by E. B. Wilson (see Scie7ice, April 

 4, 1902.) " For practical purposes cells of like prospective value, giving rise to 

 homologous structures, may, irrespective of their origin, be called equivalent ; those 

 of like ontogenetic origin and position, may, irrespective of their fate, be called 

 homoblastic ; but neither equivalent or homoblastic cells are necessarily homologous." 



