261] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 67 



The changes which take place during the fourth, fifth, and sixth days are 

 those striking in their effect upon the appearance of the embryo. During the 

 fourth day the appearance is that of a mass of cells surrounded by a thick 

 wall. It is at this period that the masses of the various organs differentiate 

 themselves and instead of a mass of cells the several tissues and organs begin 

 to be apparent. The first noticeable change is in the wall of the embryo which 

 develops into a clear, very thick layer so closely attached to the inner surface 

 of the shell that it seems to be merely a lining membrane. That this is really 

 the integument or epidermal layer of the young worm becomes evident when, 

 thru the lengthening and expanding process, this wall is folded in, away from 

 the shell. Both outer and inner boundaries of the epidermis are sharply 

 defined, so that the latter is a conspicuous portion of the embryo. The extreme 

 thickness of this layer may be explained by the cramped and contracted posi- 

 tion. During this same period the muscular system begins its growth as 

 many strands of cells attached to the firm inner wall of the integument. Thru- 

 out the following days there is an increasingly greater amount of contraction 

 and movement of the whole surface of the embryo. It is constantly being 

 spasmodically drawn in at one point or another for a minute and then slowly 

 released, as tho the different fibers were each receiving their quota of exercise. 



About this same time, also, the start of the intestine made its appearance 

 as a mass of cells nearly in the center of the egg, the connection with the mouth 

 not taking place until later. It was merely a plate-like layer of cells at first, 

 without any of the lobular ramifications of the adult organ. Little by little, 

 out thru the parenchjTua, small strands of ceUs became differentiated and 

 enlarged as branches of the main portion of the digestive tract. 



Both anterior and posterior ends began to lengthen so that the spherical 

 condition was gradually lost and the much folded, much crowded elongate 

 worm was distinguishable. The two extremities were clear and very con- 

 spicuously different from the heavy, dark middle portion. In the anterior 

 end the mass of cells which forms the brain gangHon began to take shape and 

 a little later the eyes could be seen. The two loops of the excretory tubules 

 as well as the strands of rhabdites were \dsible. There was constantly a very 

 slow rotating of the whole worm in the shell. It took sometimes from one 

 and one-half to two hours for the whole embryo to turn completely over and 

 it was rather difficult to distinguish the different parts, as the embryo was 

 much twisted. At other times, the movement was much more rapid for a 

 minute or two, and then the embryo would remain quiet for some time. 



By the morning of the seventh day the worm was slowly twisting and turn- 

 ing almost constantly, often making a complete rotation in a few seconds. 

 About this time at points where the integument was drawn away from the 

 shell, in the space thus left, long and well-developed cilia could be seen vibra- 

 ting rapidly in their close quarters. As soon as the wall returned to its position 

 against the shell, the cilia flattened down and were again invisible, altho 

 their influence must have been felt as aiding in the movement of the body as 



