628 LNVEETEBRATA CHAP. 



growth of the tongue-bars. Then another pair of slits arise on each 

 side behind the original second one (No. 4), bringing the total 

 number of protostigmata up to six. It is uncertain whether these 

 new slits (Nos. 5 and 6) arise as independent perforations of the 

 pharyngeal wall or by the bisection of a single U-shaped slit on each 

 side, but the latter hypothesis seems to be the more probable. 



Since the long axis of the pharynx in the larva becomes its 

 vertical axis in the adult, it is clear that these six protostigmata 

 must form a vertical row; and, when they become subdivided, they give 

 rise to six transverse rows of stigmata. A stage with six undivided 

 protostigmata on each side has been found in many simple Ascidians. 

 The subdivision of the protostigmata has been described by De Selys- 

 Longchamps (1901), whose account has been confirmed by Fechner 

 (1907). According to him, bud-like outgrowths are formed on the 

 bars separating Nos. 1 and 2, 3 and 4, 5 and 6, respectively, i.e. the 

 tongue-bars. The outgrowths eventually unite and form the internal 

 longitudinal bars of the adult pharynx. 



The subsequent multiplication of the stigmata of these six primary 

 transverse rows has been investigated by De Selys-Longchamps(1901), 

 and by Damas (1901) and Fechner (1907). It occurs by the transforma- 

 tion of all of them into U's, like the first-formed U's ; these have their 

 convexities towards the endostyle and their limbs pointing towards 

 the mid-dorsal line of the pharynx. Then these U's become divided 

 by dorsal downgrowths, and in this way the number of transverse 

 rows of stigmata is doubled. The number of slits in a transverse row 

 is increased by the cutting off of small portions of each slit from the 

 rest by outgrowths from the edges. The five large broad liars 

 containing blood-vessels, which separate the six primary slits, are 

 recognizable in the adult stage of those individuals which have been 

 produced, not by budding, but by the metamorphoses of larvae. 



The origin of the genital organs has not been clearly seen in 

 dona, but has been described in Molgula, another genus of simple 

 Ascidians, by De Selys-Longcharnps and Daiiias (1902). In this 

 genus, in which a remnant of the cerebral vesicle containing the otolith 

 persists throughout Me, one is able to see in the young fixed form, 

 when it has only two . gill-slits, a small rounded mass of mesoderm 

 cells on the ventral side of the pharynx beneath the bar separating 

 the two gill-slits (Fig. 457). On the left side a similar mass is seen 

 in the concavity of the last curve of the intestine. As these lobes 

 grow they become divided into a superior mass, the rudiment of the 

 ovary, and a lower one, which is the beginning of the testis. The 

 ovary is hollow; the upper wall of the cavity is flat, the lower 

 consists of the germ cells. Both rudiments grow in length and consist 

 of bands whose length is transverse to the length of the pharynx. The 

 rudiment of the testis becomes covered with rounded buds (Fig. 

 457, C), which finally constitute the ampullae where the spermatozoa 

 are produced ; ultimately the rest of the jgland becomes hollowed out 

 to form the vas defer ens. The oviduct is also formed from the 



