24 Hjalmar Théel, 



V. 

 The Gastrula stage '). 



At about twenty-four hours after the fecundation more or less, 

 according to the temperature of the water, an invagination begins to 

 take place at the vegetative pole, the invaginated portion becoming the 

 entoderm, PI. II, fig. 34 and PL III, fig. 37—38. The cavity of the 

 entoderm or archenteron, at first very minute and resembling a small 

 depression, grows rapidly and becomes deeper and deeper, taking the 

 form of a hollow cylinder. After another twelve hours the archenteron 

 has attained almost its full length, PL III, fig. 39^ the vaso-peritoneal 

 vesicles being either already separated or in a state of constriction, and 

 even at this age the Gastrula begins to pass into the first stage of the 

 Pluteus. The Gastrula, like the Blastula, is characterized by its cylin- 

 drical form and unusual length. 



The cells of the ectoderm are not closely pressed together side 

 by side, but rather distinct spindle-shaped spaces are to be traced be- 

 tween them. The cells at the animal pole are still of a marked cylindrical 

 shape and higher than the other cells. The cells at the vegetative pole 

 which also were remarkable for their length, still retain during the in- 

 vagination their cylindrical or club-shaped form and present not seldom, 

 when treated with acetic acid carmine, the characteristic stages of karyo- 

 kinesis, which show that a cell-division takes place and that the larval 



1) It has been presumed that ia all the Echinodemis the Gastrula is pro- 

 duced by an embolic invagination. However this appears not always to be the case. 

 In 1869 Metschnikoff in his »Studien über die Entwickeluug der Echinodernieu 

 and Nemertimen» alluded to certain peculiarities in the larva of Amphiura squamata, 

 stating that the cells of the Blastula divide themselves by a process of concentric 

 splitting into two layers, but he did not prosecute his researches in detail. Later 

 Apostolides (Arch. d. zool. exper. 10. 1882. p. 208) pointed out that the hypo- 

 blast originates in this A^^plii'^i'ä by delamination. Recently, Russo (Zool. Anzeig. N:o 

 377. p. 405 — 407) has declared positively that Amphiura squamata has a Gastrula 

 produced by delamination. According to him, the cylindrical blastomeres become 

 simultaneously split into an outer more plasmatic ectoderm and an inner entoderm; 

 it is not till afterwards that the two vesicles are perforated at one place and this 

 opening is known as the blastopore. 



If this be right, the continuity in the mode of the formation of the Gastrula 

 in the Echinoderms is broken in a very singular manner — but further investiga- 

 tions are needed. 



