BROOKS AND PJTTENHOUSE: ON TURRITOPSIS. 445 



tides when in the open ocean. Several times the experiment of dividing 

 the egg during the comparatively early cleavages was tried and the 

 parts were found to continue their development without any hindrance. 

 These experiments will be described more in detail later. 



Another point in which the segmenting egg of Turritopsis differs 

 from that of Oceania armata is that it does not form a true cleavage 

 cavity. The blastomeres always form a more or less solid embryo, 

 as shown in the sections of these stages. Occasionally there are small 

 spaces left between the cells; but a true segmentation cavity that later 

 forms a blastocoele is never formed. In this respect also it is similar 

 to the development of Pennaria tiarella as described by Hargitt. As 

 the completion of segmentation approaches, these irregular masses 

 of cells gradually take on a more symmetrical form and, finally, there 

 is formed an oval embryo composed of a solid mass of cells constituting 

 a morula. 



The first cleavage takes place about twenty to thirty minutes after 

 the polar bodies have been given off. It begins at the upper pole of 

 the egg and passes down to the lower pole. Thus the egg is divided 

 meridionally into two cells of approximately equal size. When 

 division is complete the blastomeres do not remain in close union, but 

 move apart so that the two spheres are connected by only small arcs 

 of their circumference. The protoplasmic bridge, which frequently 

 occurs in hydroid eggs at the lower pole just previous to the completion 

 of the two-celled stage, is usually to be seen in the egg of this species; 

 but it is much less conspicuous than is the case in Stomotoca. ^Yhen 

 it occurs, it is less definite and clearly defined than in Hydractinia, as 

 described and figured by Bunting. Metschnikoff, also, figures a very 

 beautiful example of this protoplasmic connection in the egg of Nausi- 

 thoe marginata. In Turritopsis the condition is much like that of 

 Rathkea fasciculate,, as shown by the last -mentioned observer, in which 

 the connections instead of becoming a very definite bridge remain for 

 a time as a less clearly outlined portion of the ectosarcal material. 

 Protoplasmic currents may be seen at times in these connecting fila- 

 ments. Their function does not seem to be clearly known; but it 

 very probably is connected with a readjustment of the cytoplasm 

 and the establishment of an equilibrium between the different blasto- 

 meres. 



Hargitt in his paper on "The early development of Pennaria fiarella" 

 discusses the occurrence of papillae, threads, and bridges; and reviews 



